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Register a new userDetection limits of exoplanetary atmospheres with 2-m class telescopes
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Transmission spectroscopy is an important technique to probe the atmospheres of exoplanets. With the advent of TESS and, in the future, of PLATO, more and more transiting planets around bright stars will be found and the observing time at large telescopes currently used to apply these techniques will not suffice. We demonstrate here that 2-m class telescopes equipped with spectrographs with high resolving power may be used for a certain number of potential targets. We obtained a time series of high-resolution FEROS spectra at the 2.2-m telescope at La Silla of the very hot Jupiter hosting planet WASP-18b and show that our upper limit is consistent with the expectations. This is the first analysis of its kind using 2-m class telescopes, and serves to highlight their potential. In this context, we then proceed to discuss the suitability of this class of telescopes for the upcoming flood of scientifically interesting targets from TESS space mission, and propose a methodology to select the most promising targets. This is of particular significance given that observing time on 2-m class telescopes is more readily available than on large 8-m class facilities.
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Christiane Helling (Centre for Exoplanet Science
, University of Stn Andrews
, St Andrews
2020
Today, we know ~4330 exoplanets orbiting their host stars in ~3200 planetary systems. The diversity of these exoplanets is large, and none of the known exoplanets is a twin to any of the solar system planets, nor is any of the known extrasolar planetary systems a twin of the solar system. Such diversity on many scales and structural levels requires fundamental theoretical approaches. Large efforts are underway to develop individual aspects of exoplanet sciences, like exoplanet atmospheres, cloud formation, disk chemistry, planet system dynamics, mantle convection, mass loss of planetary atmospheres. The following challenges need to be addressed in tandem with observational efforts. They provide the opportunity to progress our understanding of exoplanets and their atmospheres by exploring our models as virtual laboratories to fill gaps in observational data from different instruments and missions, and taken at different instances of times: Challenge a) Building complex models based on theoretical rigour that aim to understand the interactions of atmospheric processes, to treat cloud formation and its feedback onto the gas-phase chemistry and the energy budget of the planetary atmosphere moving away from solar-system inspired parameterisations. Challenge b) Enabling cloud modelling based on fundamental physio-chemical insights in order to be applicable to the large and unexplored chemical, radiative and thermodynamical parameter range of exoplanets in the universe. Challenge b) will be explored in this chapter of the book ExoFrontiers.
We review several aspects of the calculation of exoplanet model atmospheres in the current era, with a focus on understanding the temperature-pressure profiles of atmospheres and their emitted spectra. Most of the focus is on gas giant planets, both under strong stellar irradiation and in isolation. The roles of stellar irradiation, metallicity, surface gravity, C/O ratio, interior fluxes, and cloud opacity are discussed. Connections are made to the well-studied atmospheres of brown dwarfs as well as sub-Neptunes and terrestrial planets, where appropriate. Illustrative examples of model atmosphere retrievals on a thermal emission spectrum are given and connections are made between atmospheric abundances and the predictions of planet formation models.
We present the first exoplanet atmosphere detection made as part of the SPIRou Legacy Survey, a Large Observing Program of 300 nights exploiting the capabilities of SPIRou, the new near-infrared high-resolution (R ~ 70 000) spectro-polarimeter installed on the Canada-France-Hawaii Telescope (CFHT; 3.6-m). We observed two transits of HD 189733, an extensively studied hot Jupiter that is known to show prominent water vapor absorption in its transmission spectrum. When combining the two transits, we successfully detect the planets water vapor absorption at 5.9 sigma using a cross-correlation t-test, or with a Delta BIC >10 using a log-likelihood calculation. Using a Bayesian retrieval framework assuming a parametrized T-P profile atmosphere models, we constrain the planet atmosphere parameters, in the region probed by our transmission spectrum, to the following values: VMR[H2O] = -4.4^{+0.4}_{-0.4}, and P_cloud >~ 0.2 bar (grey clouds), both of which are consistent with previous studies of this planet. Our retrieved water volume mixing ratio is slightly sub-solar although, combining it with the previously retrieved super-solar CO abundances from other studies would imply super-solar C/O ratio. We furthermore measure a net blue shift of the planet signal of -4.62^{+0.46}_{-0.44} km s-1, which is somewhat larger than many previous measurements and unlikely to result solely from winds in the planets atmosphere, although it could possibly be explained by a transit signal dominated by the trailing limb of the planet. This large blue shift is observed in all the different detection/retrieval methods that were performed and in each of the two transits independently.
Planets can emit polarized thermal radiation, just like brown dwarfs. We present calculated thermal polarization signals from hot exoplanets, using an advanced radiative transfer code that fully includes all orders of scattering by gaseous molecules and cloud particles. The code spatially resolves the disk of the planet, allowing simulations for horizontally inhomogeneous planets. Our results show that the degree of linear polarization, P, of an exoplanets thermal radiation is expected to be highest near the planets limb and that this P depends on the temperature and its gradient, the scattering properties and the distribution of the cloud particles. Integrated over the disk of a spherically symmetric planet, P of the thermal radiation equals zero. However, for planets that appear spherically asymmetric, e.g. due to flattening, cloud bands or spots in their atmosphere, differences in their day and night sides, and/or obscuring rings, P is often larger than 0.1 %, in favorable cases even reaching several percent at near-infrared wavelengths. Detection of thermal polarization signals can give access to planetary parameters that are otherwise hard to obtain: it immediately confirms the presence of clouds, and P can then constrain atmospheric inhomogeneities and the flattening due to the planets rotation rate. For zonally symmetric planets, the angle of polarization will yield the components of the planets spin axis normal to the line-of-sight. Finally, our simulations show that P is generally more sensitive to variability in a cloudy planets atmosphere than the thermal flux is, and could hence better reveal certain dynamical processes.
The prevalence of clouds in currently observable exoplanetary atmospheres motivates the compilation and calculation of their optical properties. First, we present a new open-source Mie scattering code known as LX-MIE, which is able to consider large size parameters ($sim 10^7$) using a single computational treatment. We validate LX-MIE against the classical MIEV0 code as well as previous studies. Second, we embark on an expanded survey of the published literature for both the real and imaginary components of the refractive indices of 32 condensate species. As much as possible, we rely on experimental measurements of the refractive indices and resort to obtaining the real from the imaginary component (or vice versa), via the Kramers-Kronig relation, only in the absence of data. We use these refractive indices as input for LX-MIE to compute the absorption, scattering and extinction efficiencies of all 32 condensate species. Finally, we use a three-parameter function to provide convenient fits to the shape of the extinction efficiency curve. We show that the errors associated with these simple fits in the Wide Field Camera 3 (WFC3), J, H and K wavebands are $sim 10%$. These fits allow for the extinction cross section or opacity of the condensate species to be easily included in retrieval analyses of transmission spectra. We discuss prospects for future experimental work. The compilation of the optical constants and LX-MIE are publicly available as part of the open-source Exoclime Simulation Platform (http://www.exoclime.org).
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