R. F. Díaz, G. Montagnier, J. Leconte, A. S. Bonomo, M. Deleuil, J. M. Almenara, S. C. C. Barros, F. Bouchy, G. Bruno, C. Damiani, G. Hébrard, C. Moutou, and A. Santerne. SOPHIE velocimetry of Kepler transit candidates. XIII. KOI-189 b and KOI-686 b: two very low-mass stars in long-period orbits. Astronomy Astrophysics, 572:A109, 2014. [ bib | DOI | arXiv | PDF version | ADS link ]
We present the radial-velocity follow-up of two Kepler planetary transiting candidates (KOI-189 and KOI-686) carried out with the SOPHIE spectrograph at the Observatoire de Haute Provence. These data promptly discard these objects as viable planet candidates and show that the transiting objects are in the regime of very low-mass stars, where a strong discrepancy between observations and models persists for the mass and radius parameters. By combining the SOPHIE spectra with the Kepler light curve and photometric measurements found in the literature, we obtain a full characterization of the transiting companions, their orbits, and their host stars. The two companions are in significantly eccentric orbits with relatively long periods (30 days and 52.5 days), which makes them suitable objects for a comparison with theoretical models, since the effects invoked to understand the discrepancy with observations are weaker for these orbital distances. KOI-189 b has a mass M = 0.0745 0.0033 M&sun; and a radius R = 0.1025 0.0024 R&sun;. The density of KOI-189 b is significantly lower than expected from theoretical models for a system of its age. We explore possible explanations for this difference. KOI-189 b is the smallest hydrogen-burning star with such a precise determination of its fundamental parameters. KOI-686 b is larger and more massive (M = 0.0915 0.0043 M&sun;; R = 0.1201 0.0033 R&sun;), and its position in the mass-radius diagram agrees well with theoretical expectations.Based on observations collected with the SOPHIE spectrograph on the 1.93 m telescope at Observatoire de Haute-Provence (CNRS), France (programs 11A.PNP.MOUT and 11B.PNP.MOUT).Tables 1, 2, and 6 are available in electronic form at <A href=“http://www.aanda.org/10.1051/0004-6361/201424406/olm”>http://www.aanda.org</A>
S. Guerlet, A. Spiga, M. Sylvestre, M. Indurain, T. Fouchet, J. Leconte, E. Millour, R. Wordsworth, M. Capderou, B. Bézard, and F. Forget. Global climate modeling of Saturns atmosphere. Part I: Evaluation of the radiative transfer model. Icarus, 238:110-124, 2014. [ bib | DOI | PDF version | ADS link ]
We have developed and optimized a seasonal, radiative-convective model of Saturns upper troposphere and stratosphere. It is used to investigate Saturns radiatively-forced thermal structure between 3 and 10-6 bar, and is intended to be included in a Saturn global climate model (GCM), currently under development. The main elements of the radiative transfer model are detailed as well as the sensitivity to spectroscopic parameters, hydrocarbon abundances, aerosol properties, oblateness, and ring shadowing effects. The vertical temperature structure and meridional seasonal contrasts obtained by the model are then compared to Cassini/CIRS observations. Several significant model-observation mismatches reveal that Saturns atmosphere departs from radiative equilibrium. For instance, we find that the modeled temperature profile is close to isothermal above the 2-mbar level, while the temperature retrieved from ground-based or Cassini/CIRS data continues to increase with altitude. Also, no local temperature minimum associated to the ring shadowing is observed in the data, while the model predicts stratospheric temperatures 10 K to 20 K cooler than in the absence of rings at winter tropical latitudes. These anomalies are strong evidence that processes other that radiative heating and cooling control Saturns stratospheric thermal structure. Finally, the model is used to study the warm stratospheric anomaly triggered after the 2010 Great White Spot. Comparison with recent Cassini/CIRS observations suggests that the rapid cooling phase of this warm beacon in May-June 2011 can be explained by radiative processes alone. Observations on a longer timeline are needed to better characterize and understand its long-term evolution.
F. Forget and J. Leconte. Possible climates on terrestrial exoplanets. Philosophical Transactions of the Royal Society of London Series A, 372:20130084-20130084, 2014. [ bib | DOI | arXiv | PDF version | ADS link ]
What kind of environment may exist on terrestrial planets around other stars? In spite of the lack of direct observations, it may not be premature to speculate on exoplanetary climates, for instance to optimize future telescopic observations, or to assess the probability of habitable worlds. To first order, climate primarily depends on 1) The atmospheric composition and the volatile inventory; 2) The incident stellar flux; 3) The tidal evolution of the planetary spin, which can notably lock a planet with a permanent night side. The atmospheric composition and mass depends on complex processes which are difficult to model: origins of volatile, atmospheric escape, geochemistry, photochemistry. We discuss physical constraints which can help us to speculate on the possible type of atmosphere, depending on the planet size, its final distance for its star and the star type. Assuming that the atmosphere is known, the possible climates can be explored using Global Climate Models analogous to the ones developed to simulate the Earth as well as the other telluric atmospheres in the solar system. Our experience with Mars, Titan and Venus suggests that realistic climate simulators can be developed by combining components like a “dynamical core”, a radiative transfer solver, a parametrisation of subgrid-scale turbulence and convection, a thermal ground model, and a volatile phase change code. On this basis, we can aspire to build reliable climate predictors for exoplanets. However, whatever the accuracy of the models, predicting the actual climate regime on a specific planet will remain challenging because climate systems are affected by strong positive destabilizing feedbacks (such as runaway glaciations and runaway greenhouse effect). They can drive planets with very similar forcing and volatile inventory to completely different states.
B. Samuel, J. Leconte, D. Rouan, F. Forget, A. Léger, and J. Schneider. Constraining physics of very hot super-Earths with the James Webb Telescope. The case of CoRot-7b. Astronomy Astrophysics, 563:A103, 2014. [ bib | DOI | arXiv | PDF version | ADS link ]
Context. Transit detection from space using ultra-precise photometry led to the first detection of super-Earths with solid surfaces: CoRot-7b and Kepler-10b. Because they lie only a few stellar radii from their host stars, these two rocky planets are expected to be extremely hot. <BR /> Aims: Assuming that these planets are in a synchronous rotation state and receive strong stellar winds and fluxes, previous studies have suggested that they must be atmosphere-free and that a lava ocean is present on their hot dayside. In this article, we use several dedicated thermal models of the irradiated planet to study how observations with NIRSPEC on the James Webb Space Telescope (JWST) could further confirm and constrain, or reject the atmosphere-free lava ocean planet model for very hot super-Earths. <BR /> Methods: Using CoRoT-7b as a working case, we explore the consequences on the phase-curve of a non tidal-locked rotation, with the presence/absence of an atmosphere, and for different values of the surface albedo. We then simulate future observations of the reflected light and thermal emission from CoRoT-7b with NIRSPEC-JWST and look for detectable signatures, such as time lag, of those peculiarities. We also study the possibility to retrieve the latitudinal surface temperature distribution from the observed SED. <BR /> Results: We demonstrate that we should be able to constrain several parameters after observations of two orbits (42 h) thanks to the broad range of wavelengths accessible with JWST: i) the Bond albedo is retrieved to within 0.03 in most cases. ii) The lag effect allows us to retrieve the rotation period within 3 h of a non phase-locked planet, whose rotation would be half the orbital period; for longer period, the accuracy is reduced. iii) Any spin period shorter than a limit in the range 30-800 h, depending on the thickness of the thermal layer in the soil, would be detected. iv) The presence of a thick gray atmosphere with a pressure of one bar, and a specific opacity higher than 10-5 m-2 kg-1 is detectable. v) With spectra up to 4.5 μm, the latitudinal temperature profile can be retrieved to within 30 K with a risk of a totally wrong solution in 5% of the cases. This last result is obtained for a signal-to-noise ratio around 5 per resel, which should be reached on Corot-7 after a total exposure time of ˜70 h with NIRSPEC and only three hours on a V = 8 star. <BR /> Conclusions: We conclude that it should thus be possible to distinguish the reference situation of a lava ocean with phase-locking and no atmosphere from other cases. In addition, obtaining the surface temperature map and the albedo brings important constraints on the nature or the physical state of the soil of hot super-Earths.