Searching for the Transit of the Earth–mass exoplanet Proxima~Centauri~b in Antarctica: Preliminary Result [EPA]

Proxima~Centauri is known as the closest star from the Sun. Recently, radial velocity observations revealed the existence of an Earth–mass planet around it. With an orbital period of $\sim$11 days, the surface of Proxima Centauri b is temperate and might be habitable. We took a photometric monitoring campaign to search for its transit, using the Bright Star Survey Telescope at the Zhongshan Station in Antarctica. A transit–like signal appearing on September 8th, 2016, is identified tentatively. Its midtime, $T_{C}=2,457,640.1990\pm0.0017$~HJD, is consistent with the predicted ephemeris based on RV orbit in a 1$\sigma$ confidence interval. Time–correlated noise is pronounced in the light curve of Proxima Centauri, affecting detection of transits. We develop a technique, in a Gaussian process framework, to gauge the statistical significance of potential transit detection. The tentative transit signal reported here, has a confidence level of $2.5\sigma$. Further detection of its periodic signals is necessary to confirm the planetary transit of Proxima Centauri b. We plan to monitor Proxima Centauri in next Polar night at Dome A in Antarctica, taking the advantage of continuous darkness. \citet{Kipping17} reported two tentative transit–like signals of Proxima Centauri b, observed by the Microvariability and Oscillation of Stars space Telescope in 2014 and 2015, respectively. The midtransit time of our detection is 138 minutes later than that predicted by their transit ephemeris. If all the signals are real transits, the misalignment of the epochs plausibly suggests transit timing variations of Proxima Centauri b induced by an outer planet in this system.

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H. Liu, P. Jiang, X. Huang, et. al.
Tue, 21 Nov 17

Comments: 17 pages, 8 figures, 2 tables, accepted by The Astronomical Journal

Lyα Absorption at Transits of HD 209458b: A Comparative Study of Various Mechanisms Under Different Conditions [EPA]

To shed more light on the nature of the observed Ly{\alpha} absorption during transits of HD 209458b and to quantify the major mechanisms responsible for the production of fast hydrogen atoms (the so called energetic neutral atoms, ENAs) around the planet, 2D hydrodynamic multifluid modeling of the expanding planetary upper atmosphere, which is driven by stellar XUV, and its interaction with the stellar wind has been performed. The model selfconsistently describes the escaping planetary wind, taking into account the generation of ENAs due to particle acceleration by the radiation pressure and by the charge exchange between the stellar wind protons and planetary atoms. The calculations in a wide range of stellar wind parameters and XUV flux values showed that under typical Sun-like star conditions, the amount of generated ENAs is too small, and the observed absorption at the level of 6-8 percent can be attributed only to the non-resonant natural line broadening. For lower XUV fluxes, e.g., during the activity minima, the number of planetary atoms that survive photoionization and give rise to ENAs increases, resulting in up to 10-15 percent absorption at the blue wing of the Lya line, caused by resonant thermal line broadening. A similar asymmetric absorption can be seen under the conditions realized during coronal mass ejections, when sufficiently high stellar wind pressure confines the escaping planetary material within a kind of bowshock around the planet. It was found that the radiation pressure in all considered cases has a negligible contribution to the production of ENAs and the corresponding absorption.

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M. Khodachenko, I. Shaikhislamov, H. Lammer, et. al.
Tue, 21 Nov 17

Comments: open access

Detection of planet candidates around K giants, HD 40956, HD 111591, and HD 113996 [EPA]

Aims. The purpose of this paper is to detect and investigate the nature of long-term radial velocity (RV) variations of K-type giants and to confirm planetary companions around the stars.
Methods. We have conducted two planet search programs by precise RV measurement using the 1.8 m telescope at Bohyunsan Optical Astronomy Observatory (BOAO) and the 1.88 m telescope at Okayama Astrophysical Observatory (OAO). The BOAO program searches for planets around 55 early K giants. The OAO program is looking for 190 G-K type giants.
Results. In this paper, we report the detection of long-period RV variations of three K giant stars, HD 40956, HD 111591, and HD 113996. We investigated the cause of the observed RV variations and conclude the substellar companions are most likely the cause of the RV variations. The orbital analyses yield P = 578.6 $\pm$ 3.3 d, $m$ sin $i$ = 2.7 $\pm$ 0.6 $M_{\rm{J}}$, $a$ = 1.4 $\pm$ 0.1 AU for HD 40956; P = 1056.4 $\pm$ 14.3 d, $m$ sin $i$ = 4.4 $\pm$ 0.4 $M_{\rm{J}}$, $a$ = 2.5 $\pm$ 0.1 AU for HD 111591; P = 610.2 $\pm$ 3.8 d, $m$ sin $i$ = 6.3 $\pm$ 1.0 $M_{\rm{J}}$, $a$ = 1.6 $\pm$ 0.1 AU for HD 113996.

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G. Jeong, B. Lee, I. Han, et. al.
Tue, 21 Nov 17

Comments: 10 pages, 12 figures. Accepted for publication in Astronomy & Astrophysics

A survey of eight hot Jupiters in secondary eclipse using WIRCam at CFHT [EPA]

We present near infrared high-precision photometry for eight transiting hot Jupiters observed during their predicted secondary eclipses. Our observations were carried out using the staring mode of the WIRCam instrument on the Canada-France-Hawaii Telescope (CFHT). We present the observing strategies and data reduction methods which delivered time series photometry with statistical photometric precisionas low as 0.11%. We performed a Bayesian analysis to model the eclipse parameters and systematics simultaneously. The measured planet-to-star flux ratios allowed us to constrain the thermal emission from the day side of these hot Jupiters, as we derived the planet brightness temperatures. Our results combined with previously observed eclipses reveal an excess in the brightness temperatures relative to the blackbody prediction for the equilibrium temperatures of the planets for a wide range of heat redistribution factors. We find a trend that this excess appears to be larger for planets with lower equilibrium temperatures. This may imply some additional sources of radiation, such as reflected light from the host star and/or thermal emission from residual internal heat from the formation of the planet.

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E. Martioli, K. Colon, D. Angerhausen, et. al.
Tue, 21 Nov 17

Comments: 17 pages, 19 figures, accepted for publication in the Monthly Notices of the Royal Astronomical Society

Numerical solution of a non-linear conservation law applicable to the interior dynamics of partially molten planets [EPA]

The energy balance of a partially molten rocky planet can be expressed as a non-linear diffusion equation using mixing length theory to quantify heat transport by both convection and mixing of the melt and solid phases. In this formulation the effective or eddy diffusivity depends on the entropy gradient, $\partial S/\partial r$, as well as entropy. First we present a simplified model with semi-analytical solutions, highlighting the large dynamic range of $\partial S/\partial r$, around 12 orders of magnitude, for physically-relevant parameters. It also elucidates the thermal structure of a magma ocean during the earliest stage of crystal formation. This motivates the development of a simple, stable numerical scheme able to capture the large dynamic range of $\partial S/\partial r$ and provide a flexible and robust method for time-integrating the energy equation.
We then consider a full model including energy fluxes associated with convection, mixing, gravitational separation, and conduction that all depend on the thermophysical properties of the melt and solid phases. This model is discretised and evolved by applying the finite volume method (FVM), allowing for extended precision calculations and using $\partial S/\partial r$ as the solution variable. The FVM is well-suited to this problem since it is naturally energy conserving, flexible, and intuitive to incorporate arbitrary non-linear fluxes that rely on lookup data. Special attention is given to the numerically challenging scenario in which crystals first form in the centre of a magma ocean.
Our computational framework is immediately applicable to modelling high melt fraction phenomena in Earth and planetary science research. Furthermore, it provides a template for solving similar non-linear diffusion equations arising in other disciplines, particularly for non-linear functional forms of the diffusion coefficient.

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D. Bower, P. Sanan and A. Wolf
Tue, 21 Nov 17

Comments: N/A

Vortices and the saturation of the vertical shear instability in protoplanetary disks [EPA]

If sufficiently irradiated by its central star, a protoplanetary disks falls into an equilibrium state exhibiting vertical shear. This state may be subject to a hydrodynamical instability, the vertical shear instability' (VSI), whose breakdown into turbulence transports a moderate amount of angular momentum while also facilitating planet formation, possibly via the production of small-scale vortices. In this paper, we show that VSI modes (a) exhibit arbitrary spatial profiles and (b) remain nonlinear solutions to the incompressible local equations, no matter their amplitude. The modes are themselves subject to parasitic Kelvin-Helmholtz instability, though the disk rotation significantly impedes the parasites and permits the VSI to attain large amplitudes (fluid velocities < 10% the sound speed). Thisdelay’ in saturation probably explains the prominence of the VSI linear modes in global simulations. More generally, the parasites may set the amplitude of VSI turbulence in strongly irradiated disks. They are also important in breaking the axisymmetry of the flow, via the unavoidable formation of vortices. The vortices, however, are not aligned with the orbital plane and thus express a pronounced $z$-dependence. We also briefly demonstrate that the vertical shear has little effect on the magnetorotational instability, whereas magnetic fields easily quench the VSI, a potential issue in the ionised surface regions of the disk and also at larger radii.

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H. Latter and J. Papaloizou
Tue, 21 Nov 17

Comments: 16 pages, MNRAS accepted

FUV Spectral Signatures of Molecules and the Evolution of the Gaseous Coma of Comet 67P/Churyumov-Gerasimenko [EPA]

The Alice far-ultraviolet imaging spectrograph onboard Rosetta observed emissions from atomic and molecular species from within the coma of comet 67P/Churyumov-Gerasimenko during the entire escort phase of the mission from 2014 August to 2016 September. The initial observations showed that emissions of atomic hydrogen and oxygen close to the surface were produced by energetic electron impact dissociation of H2O. Following delivery of the lander, Philae, on 2014 November 12, the trajectory of Rosetta shifted to near-terminator orbits that allowed for these emissions to be observed against the shadowed nucleus that, together with the compositional heterogeneity, enabled us to identify unique spectral signatures of dissociative electron impact excitation of H2O, CO2, and O2. CO emissions were found to be due to both electron and photoexcitation processes. Thus we are able, from far-ultraviolet spectroscopy, to qualitatively study the evolution of the primary molecular constituents of the gaseous coma from start to finish of the escort phase. Our results show asymmetric outgassing of H2O and CO2 about perihelion, H2O dominant before and CO2 dominant after, consistent with the results from both the in situ and other remote sensing instruments on Rosetta.

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P. Feldman, M. AHearn, J. Bertaux, et. al.
Tue, 21 Nov 17

Comments: 11 pages, 8 figures, accepted for publication in the Astronomical Journal