Constraints on the Magnetic Field Strength of HAT-P-7 b and other Hot Giant Exoplanets [EPA]

Observations of infrared and optical light curves of hot Jupiters have demonstrated that the peak brightness is generally offset eastward from the substellar point [1,2]. This observation is consistent with hydrodynamic numerical simulations that produce fast, eastward directed winds which advect the hottest point in the atmosphere eastward of the substellar point [3,4]. However, recent continuous Kepler measurements of HAT-P-7 b show that its peak brightness offset varies significantly in time, with excursions such that the brightest point is sometimes westward of the substellar point [5]. These variations in brightness offset require wind variability, with or without the presence of clouds. While such wind variability has not been seen in hydrodynamic simulations of hot Jupiter atmospheres, it has been seen in magnetohydrodynamic (MHD) simulations [6]. Here we show that MHD simulations of HAT-P-7 b indeed display variable winds and corresponding variability in the position of the hottest point in the atmosphere. Assuming the observed variability in HAT-P-7 b is due to magnetism we constrain its minimum magnetic field strength to be 6\,G. Similar observations of wind variability on hot giant exoplanets, or lack thereof, could help constrain their magnetic field strengths. Since dynamo simulations of these planets do not exist and theoretical scaling relations [7] may not apply, such observational constraints could prove immensely useful.

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T. Rogers
Mon, 24 Apr 17

Comments: 8 pages, 3 figures, Accepted at Nature Astronomy

Spectra and physical properties of Taurid meteoroids [EPA]

Taurids are an extensive stream of particles produced by comet 2P/Encke, which can be observed mainly in October and November as a series of meteor showers rich in bright fireballs. Several near-Earth asteroids have also been linked with the meteoroid complex, and recently the orbits of two carbonaceous meteorites were proposed to be related to the stream, raising interesting questions about the origin of the complex and the composition of 2P/Encke. Our aim is to investigate the nature and diversity of Taurid meteoroids by studying their spectral, orbital, and physical properties determined from video meteor observations. Here we analyze 33 Taurid meteor spectra captured during the predicted outburst in November 2015 by stations in Slovakia and Chile, including 14 multi-station observations for which the orbital elements, material strength parameters, dynamic pressures, and mineralogical densities were determined. It was found that while orbits of the 2015 Taurids show similarities with several associated asteroids, the obtained spectral and physical characteristics point towards cometary origin with highly heterogeneous content. Observed spectra exhibited large dispersion of iron content and significant Na intensity in all cases. The determined material strengths are typically cometary in the $K_B$ classification, while $P_E$ criterion is on average close to values characteristic for carbonaceous bodies. The studied meteoroids were found to break up under low dynamic pressures of 0.02 – 0.10 MPa, and were characterized by low mineralogical densities of 1.3 – 2.5 g cm$^{-3}$. The widest spectral classification of Taurid meteors to date is presented.

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P. Matlovic, J. Toth, R. Rudawska, et. al.
Mon, 24 Apr 17

Comments: N/A

An Optical/near-infrared investigation of HD 100546 b with the Gemini Planet Imager and MagAO [EPA]

We present H band spectroscopic and Halpha photometric observations of HD 100546 obtained with GPI and MagAO. We detect H band emission at the location of the protoplanet HD 100546b, but show that choice of data processing parameters strongly affects the morphology of this source. It appears point-like in some aggressive reductions, but rejoins an extended disk structure in the majority of the others. Furthermore, we demonstrate that this emission appears stationary on a timescale of 4.6 yrs, inconsistent at the 2sigma level with a Keplerian clockwise orbit at 59 au in the disk plane. The H band spectrum of the emission is inconsistent with any type of low effective temperature object or accreting protoplanetary disk. It strongly suggests a scattered light origin, as it is consistent with the spectrum of the star and the spectra extracted at other locations in the disk. A non detection at the 5sigma level of HD 100546b in differential Halpha imaging places an upper limit, assuming the protoplanet lies in a gap free of extinction, on the accretion luminosity and accretion rate of 1.7E-4 Lsun and MMdot<6.4E-7Mjup^2/yr for 1Rjup. These limits are comparable to the accretion luminosity and rate of TTauri-stars or LkCa 15b. Taken together, these lines of evidence suggest that the H band source at the location of HD 100546b is not emitted by a planetary photosphere or an accreting circumplanetary disk but is a disk feature enhanced by the PSF subtraction process. This non-detection is consistent with the non-detection in the K band reported in an earlier study but does not exclude the possibility that HD 100546b is deeply embedded.

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J. Rameau, K. Follette, L. Pueyo, et. al.
Mon, 24 Apr 17

Comments: 10 pages, 5 figures, 1 table

The origin of the occurrence rate profile of gas giants inside 100 days [EPA]

We investigate the origin of the period distribution of giant planets. We try to fit the bias-corrected distribution of giant planets inside 300 days found by Santerne et al. (2016) using a planet formation model based on pebble accretion. We investigate two possible initial conditions: a linear distribution of planetary seeds, and seeds injected exclusively on the water and CO icelines. Our simulations exclude the linear initial distribution of seeds with a high degree of confidence. Our bimodal model based on snowlines give a more reasonable fit to the data, with the discrepancies reducing sig- nificantly if we assume the water snowline to be a factor 3-10 less efficient at producing planetary seeds. This model moreover performs better on both the warm/hot Jupiters ratio and the Gaussian mixture model as comparison criteria. Our results hint that the giant exoplanets population inside 300 days is more compatible with planets forming preferentially at special locations.

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M. Ali-Dib, A. Johansen and C. Huang
Mon, 24 Apr 17

Comments: 8 pages, 6 figures, submitted to MNRAS (revised version after the referee’s first report)

Planetary migration and the origin of the 2:1 and 3:2 (near)-resonant population of close-in exoplanets [EPA]

We present an analytical and numerical study of the orbital migration and resonance capture of fictitious two-planet systems with masses in the super-Earth range undergoing Type-I migration. We find that, depending on the flare index and proximity to the central star, the average value of the period ratio, $P_2/P_1$, between both planets may show a significant deviation with respect to the nominal value. For planets trapped in the 2:1 commensurability, offsets may reach values on the order of $0.1$ for orbital periods on the order of $1$ day, while systems in the 3:2 mean-motion resonance (MMR) show much smaller offsets for all values of the semimajor axis. These properties are in good agreement with the observed distribution of near-resonant exoplanets, independent of their detection method. We show that 2:1-resonant systems far from the star, such as HD82943 and HR8799, are characterized by very small resonant offsets, while higher values are typical of systems discovered by Kepler with orbital periods approximately a few days. Conversely, planetary systems in the vicinity of the 3:2 MMR show little offset with no significant dependence on the orbital distance. In conclusion, our results indicate that the distribution of Kepler planetary systems around the 2:1 and 3:2 MMR are consistent with resonant configurations obtained as a consequence of a smooth migration in a laminar flared disk, and no external forces are required to induce the observed offset or its dependence with the commensurability or orbital distance from the star.

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X. Ramos, C. Charalambous, P. Benitez-Llambay, et. al.
Mon, 24 Apr 17

Comments: 11 pages, 7 figures, accepted for publication in A&A

Complex Spiral Structure in the HD 100546 Transitional Disk as Revealed by GPI and MagAO [EPA]

We present optical and near-infrared high contrast images of the transitional disk HD 100546 taken with the Magellan Adaptive Optics system (MagAO) and the Gemini Planet Imager (GPI). GPI data include both polarized intensity and total intensity imagery, and MagAO data are taken in Simultaneous Differential Imaging mode at H{\alpha}. The new GPI H -band total intensity data represent a significant enhancement in sensitivity and field rotation compared to previous data sets and enable a detailed exploration of substructure in the disk. The data are processed with a variety of differential imaging techniques (polarized, angular, reference, and simultaneous differential imaging) in an attempt to identify the disk structures that are most consistent across wavelengths, processing techniques, and algorithmic parameters. The inner disk cavity at 15 au is clearly resolved in multiple datasets, as are a variety of spiral features. While the cavity and spiral structures are identified at levels significantly distinct from the neighboring regions of the disk under several algorithms and with a range of algorithmic parameters, emission at the location of HD 100546 c varies from point-like under aggressive algorithmic parameters to a smooth continuous structure with conservative parameters, and is consistent with disk emission. Features identified in the HD100546 disk bear qualitative similarity to computational models of a moderately inclined two-armed spiral disk, where projection effects and wrapping of the spiral arms around the star result in a number of truncated spiral features in forward-modeled images.

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K. Follette, J. Rameau, R. Dong, et. al.
Mon, 24 Apr 17

Comments: accepted to AJ

Applicability and limits of simple hydrodynamic scaling for collisions of water-rich bodies in different mass regimes [EPA]

We investigate the outcome of collisions in very different mass regimes, but an otherwise identical parameter setup, comprising the impact velocity ($v/v_\mathrm{esc}$), impact angle, mass ratio, and initial composition, w.r.t. simple hydrodynamic scaling. The colliding bodies’ masses range from $\simeq 10^{16}$ to $10^{24}$ kg, which includes km-sized planetesimals up to planetary-sized objects. Our analysis of the results comprises the time evolution of fragment masses, the fragments’ water contents and fragment dynamics, where we start with bodies consisting of basalt and water ice. The usual assumption of hydrodynamic scaling over a wider range of masses is based on material behavior similar to a fluid, or a rubble pile, respectively. All our simulations are carried out once including full solid-body physics, and once for strengthless – but otherwise identical – bodies, to test for the influence of material strength.
We find that scale-invariance over a wider range of masses is mostly only a very crude approximation at best, but can be applied to constrained mass ranges if tested carefully. For the chosen scenarios the outcomes of solid-body objects compared to strengthless fluid bodies differ most for our intermediate masses, but are similar for the lowest and highest masses. The most energetic, planet-sized collisions produce considerably faster and more fragments, which is also reflected in high water losses – above 50 percent in a single collision.

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C. Burger and C. Schafer
Fri, 21 Apr 17

Comments: Proceedings of the First Greek-Austrian Workshop on Extrasolar Planetary Systems, Ammouliani, Greece, Sept. 2016. Editors: Thomas I. Maindl, Harry Varvoglis, Rudolf Dvorak – 19 pages, 9 figures