Jupiter's North Equatorial Belt expansion and thermal wave activity ahead of Juno's arrival [EPA]


The dark colors of Jupiter’s North Equatorial Belt (NEB, $7-17^\circ$N) appeared to expand northward into the neighboring zone in 2015, consistent with a 3-5 year cycle of activity in the NEB. Inversions of thermal-IR imaging from the Very Large Telescope revealed a moderate warming and reduction of aerosol opacity at the cloud tops at $17-20^\circ$N, suggesting subsidence and drying in the expanded sector. Two new thermal waves were identified during this period: (i) an upper tropospheric thermal wave (wavenumber 16-17, amplitude 2.5 K at 170 mbar) in the mid-NEB that was anti-correlated with haze reflectivity; and (ii) a stratospheric wave (wavenumber 13-14, amplitude 7.3 K at 5 mbar) at $20-30^\circ$N. Both were quasi-stationary, confined to regions of eastward zonal flow, and are morphologically similar to waves observed during previous expansion events.

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L. Fletcher, G. Orton, J. Sinclair, et. al.
Fri, 18 Aug 17

Comments: 28 pages, 15 figures, published in Geophysical Research Letters

Cycles of Activity in the Jovian Atmosphere [EPA]


Jupiter’s banded appearance may appear unchanging to the casual observer, but closer inspection reveals a dynamic, ever-changing system of belts and zones with distinct cycles of activity. Identification of these long-term cycles requires access to datasets spanning multiple jovian years, but explaining them requires multi-spectral characterization of the thermal, chemical, and aerosol changes associated with visible color variations. The Earth-based support campaign for Juno’s exploration of Jupiter has already characterized two upheaval events in the equatorial and temperate belts that are part of long-term jovian cycles, whose underlying sources could be revealed by Juno’s exploration of Jupiter’s deep atmosphere.

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L. Fletcher
Fri, 18 Aug 17

Comments: 9 pages, 1 figures, Commentary published in Geophysical Research Letters

Adaptive optics and lightcurve data of asteroids: twenty shape models and information content analysis [EPA]


We present shape models and volume estimates of twenty asteroids based on relative photometry and adaptive optics images. We discuss error estimation and the effects of myopic deconvolution on shape solutions. For further analysis of the information capacities of data sources, we also present and discuss ambiguity and uniqueness results for the reconstruction of nonconvex shapes from photometry.

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M. Viikinkoski, J. Hanus, M. Kaasalainen, et. al.
Fri, 18 Aug 17

Comments: Accepted to A&A

Searching for Faint Comoving Companions to the $α$ Centauri system in the VVV Survey Infrared Images [SSA]


The VVV survey has observed the southern disk of the Milky Way in the near infrared, covering 240 deg$^{2}$ in the $ZYJHK_S$ filters. We search the VVV Survey images in a $\sim$19 deg$^{2}$ field around $\alpha$ Centauri, the nearest stellar system to the Sun, to look for possible overlooked companions that the baseline in time of VVV would be able to uncover. The photometric depth of our search reaches $Y\sim$19.3 mag, $J\sim$19 mag, and $K_S\sim$17 mag. This search has yielded no new companions in $\alpha$ Centauri system, setting an upper mass limit for any unseen companion well into the brown dwarf/planetary mass regime. The apparent magnitude limits were turned into effective temperature limits, and the presence of companion objects with effective temperatures warmer than 325K can be ruled out using different state-of-the-art atmospheric models.
These limits were transformed into mass limits using evolutionary models, companions with masses above 11 M$_{Jup}$ were discarded, extending the constraints recently provided in the literature up to projected distances of d<7 000 AU from $\alpha$ Cen AB and $\sim$1200 AU from Proxima. In the next few years, the VVV extended survey (VVVX) will allow to extend the search and place similar limits on brown dwarfs/planetary companions to $\alpha$ Cen AB for separations up to 20 000AU.

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J. Beamin, D. Minniti, J. Pullen, et. al.
Fri, 18 Aug 17

Comments: 8 pages, 6 figures, 3 tables. Accepted for publication in MNRAS

The Unusual Apparition of Comet 252P/2000 G1 (LINEAR) and Comparison with Comet P/2016 BA14 (PanSTARRS) [EPA]


We imaged Comet 252P/2000 G1 (LINEAR) (hereafter 252P) with the Hubble Space Telescope and both 252P and P/2016 BA${14}$ (PanSTARRS) (hereafter BA${14}$) with the Discovery Channel Telescope in March and April 2016, surrounding its close encounter to Earth. The r’-band $Af\rho$ of 252P in a 0.2″-radius aperture were $16.8\pm0.3$ and $57\pm1$ cm on March 14 and April 4, respectively, and its gas production rates were: $Q$(OH) = $(5.8\pm0.1)\times10^{27}$ s$^{-1}$, and $Q$(CN) = $(1.25\pm0.01)\times10^{25}$ s$^{-1}$ on April 17. The r’-band upper limit $Af\rho$ of BA1$_{14}$ was $0.19\pm0.01$ cm in a 19.2″-radius aperture, and $Q$(CN) = $(1.4\pm0.1)10^{22}$ s$^{-1}$ on April 17, 2017. 252P shows a bright and narrow jet of a few hundred kilometers long in the sunward direction, changing its projected position angle in the sky with a periodicity consistent with 7.24 hours. However, its photometric lightcurve is consistent with a periodicity of 5.41 hours. We suggest that the nucleus of 252P is likely in a non-principal axis rotation. The nucleus radius of 252P is estimated to be about $0.3\pm0.03$ km, indicating an active fraction of 40% to >100% in its 2016 apparition. Evidence implies a possible cloud of slow-moving grains surrounding the nucleus. The activity level of 252P in the 2016 apparition increased by two orders of magnitude from its previous apparitions, making this apparition unusual. On the other hand, the activity level of BA14 appears to be at least three orders of magnitude lower than that of 252P, despite its ten times or larger surface area.

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J. Li, M. Kelley, N. Samarasinha, et. al.
Fri, 18 Aug 17

Comments: 31 pages, 15 figures, 4 tables, accepted by AJ

Effects of Planetesimal Accretion on the Thermal and Structural Evolution of Sub-Neptunes [EPA]


A remarkable discovery from NASA’s Kepler mission is the wide diversity in the average densities of planets that defy traditional planet formation theories. After the dissipation of gas disk, planets could interact with nearby planetesimals from a remnant planetesimal disk. Typical low planetary densities, their proximity to the host stars, and the high star-planet mass ratios often lead to planetesimal accretion by the planets as a result of these interactions. Here we present calculations using the open-source stellar evolution toolkit MESA (Modules for Experiments in Stellar Astrophysics) modified to include the deposition of planetesimals into the hydrogen/helium envelopes of sub-Neptunes (~sim 1-20 MEarth). We show that such accretion can alter the mass-radius isochrones for these planets. Due to the inherent stochasticity, the same initial planet as a result of the same total accreted mass of planetesimals can have average densities different by up to ~5% several ~Gyr after the last accretion. Of course, during the active accretion period these differences are more dramatic. Furthermore, the additional energy deposition from the accreted planetesimals increases the ratio between the planet’s radius to that of the core, which in turn leads to enhanced loss of atmospheric mass. Planet mean densities, long after the phase of high accretion rate are also altered by planetesimal accretion, especially for planets initially less massive than ~10 MEarth and with envelope mass fraction less than ~10%. These results indicate that planetesimal accretion may be an important contributor in shaping the properties and diverse densities of sub-Neptune sized planets in addition to the more well-studied effects of photo-evaporation.

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S. Chatterjee and H. Chen
Fri, 18 Aug 17

Comments: 17 Pages, 9 Figures, 1 Table; Submitted to the Astrophysical Journal

Harvesting the decay energy of $^{26}$Al to drive lightning discharge in protoplanetary discs [EPA]


Chondrules in primitive meteorites likely formed by recrystallisation of dust aggregates that were flash-heated to nearly complete melting. Chondrules may represent the building blocks of rocky planetesimals and protoplanets in the inner regions of protoplanetary discs, but the source of ubiquitous thermal processing of their dust aggregate precursors remains elusive. Here we demonstrate that escape of positrons released in the decay of the short-lived radionuclide $^{26}$Al leads to a large-scale charging of dense pebble structures, resulting in neutralisation by lightning discharge and flash-heating of dust and pebbles. This charging mechanism is similar to a nuclear battery where a radioactive source charges a capacitor. We show that the nuclear battery effect operates in circumplanetesimal pebble discs. The extremely high pebble densities in such discs are consistent with conditions during chondrule heating inferred from the high abundance of sodium within chondrules. The sedimented mid-plane layer of the protoplanetary disc may also be prone to charging by the emission of positrons, if the mass density of small dust there is at least an order of magnitude above the gas density. Our results imply that the decay energy of $^{26}$Al can be harvested to drive intense lightning activity in protoplanetary discs. The total energy stored in positron emission is comparable to the energy needed to melt all solids in the protoplanetary disc. The efficiency of transferring the positron energy to the electric field nevertheless depends on the relatively unknown distribution and scale-dependence of pebble density gradients in circumplanetesimal pebble discs and in the protoplanetary disc mid-plane layer.

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A. Johansen and S. Okuzumi
Fri, 18 Aug 17

Comments: Submitted to Astronomy & Astrophysics, 22 pages, revised version in response to referee report