Oral presentations and abstracts
Session assets
The sodium doublet is one of the most powerful probes of exoplanet atmospheric properties when observed in transmission spectroscopy during transits. Recent high-spectral resolution observations of the sodium doublet in hot gas giants allowed us to resolve the line shape, opening the way for extracting atmospheric properties using line-profile fitting.
Using the MERC code (Seidel et al. 2020a), a retrieval tool to determine temperature-pressure profiles and high-altitude winds in exoplanet thermospheres, we have studied the curiously broadened sodium signatures of various hot Jupiters. We have updated the MERC code to a quasi 3D treatment of the atmosphere (Seidel et al. 2020c, in prep.) and analysed three hot Jupiters, spanning a wide range of this class of exoplanets (see figure). Using the sodium signature of three examples - WASP-76b (a highly irradiated ultra-hot Jupiter, Seidel et al. 2019), KELT-11b (a puffy hot Jupiter, Mounzer et al. 2020, in prep.), and lastly HD189733b (one of the most studied hot Jupiters to date, Wyttenbach et al. 2015) - we explore possible trends in the atmospheric structure of hot Jupiters.
We will first introduce the new quasi 3D retrieval of MERC, and proceed to show that high-velocity winds in the thermosphere are one possible explanation of the broadened sodium features seen in hot Jupiters. We plan to highlight various caveats and present likely origin scenarios for the observed wind patterns. We will then put these results in the context of past studies using global circulation models (GCMs) on hot Jupiters.
How to cite: Seidel, J., Ehrenreich, D., Bourrier, V., Pino, L., Wyttenbach, A., Allart, R., Lavie, B., Mounzer, D., and Lovis, C.: Wind of Change: Atmospheric wind retrieval and its implications for hot Jupiters, Europlanet Science Congress 2020, online, 21 Sep–9 Oct 2020, EPSC2020-273, https://doi.org/10.5194/epsc2020-273, 2020.
Motivation
The transmission spectra of many hot Jupiters show signatures of high-altitude aerosols [e.g., 1,2]. One hypothesized formation mechanism for these aerosols is that photochemical processes generate hazes on the dayside. All previous studies of photochemical hazes on tidally locked giant planets used one-dimensional models [e.g., 3,4]. However, one-dimensional models have to make strongly simplifying assumptions about the strength of vertical mixing. Furthermore, they ignore that the strong day-night contrast on hot Jupiters and atmospheric circulation can lead to inhomogeneous aerosol distributions. For condensate clouds, it has been shown that inhomogeneous aerosol distributions are to be expected [5-7] and can lead to biases in the interpretation of observations [e.g., 8]. The same is likely to be true for photochemical hazes. Further, it has been suggested differences between the morning and evening terminator in ingress and egress transmission spectra could provide a diagnostic for distinguishing between condensate clouds and photochemical hazes [9]. Three-dimensional general circulation models (GCMs) are needed to study how atmospheric circulation shapes the distribution of photochemical hazes to guide future observations and models.
Methods
We present simulations of hot Jupiter HD 189733b using the MITgcm [10]. We use passive tracers representing photochemical hazes to study how hazes are transported by atmospheric circulation. Haze particles in our model have a constant size and are spherical.
Results
The results show that the haze mass mixing ratio varies horizontally by at least an order of magnitude for all particle sizes considered and over the entire simulated pressure range. Depending on the particle size, the resulting 3D haze distribution falls into one of two regimes: small (<30 nm) and large (>30 nm) particles.
For small particles (< 30 nm), the timescale for gravitational settling is longer than the timescale for horizontal and vertical advection. The 3D distribution is thus controlled by advection and looks similar for all particle sizes in this regime. At low pressures, small particle hazes accumulate on the night side in two large midlatitude vortices centered east of the antistellar point (Fig. 1). Because the night side vortices span across the morning terminator, there are higher mass mixing ratios at the morning terminator compared to the evening terminator.
For large particles (>30 nm), the 3D haze distribution is strongly influenced by settling. At very low pressures, where the settling timescale is much shorter than the advection timescale, the horizontal pattern closely mirrors the haze production function. At somewhat higher pressures, where both timescales are within an order of magnitude from each other, hazes are concentrated on the dayside and the hemisphere east of the substellar point (Fig. 2). This results in higher mass mixing ratios at the evening terminator compared to the morning terminator. Because the distribution of hazes is dependent on where in the atmosphere these timescales become equal, the 3D size distributions look much less similar between different particle sizes within this regime compared to the small particle regime.
At pressures > 1 mbar, the advection time scale is shorter than the settling time scale for all particle sizes considered in our simulations. In this region, the equatorial jet dominates the atmospheric circulation and hazes of all sizes develop a more banded pattern (Fig. 3). Differences between morning and evening terminator become smaller in this pressure range.
Our model does not include particle growth and thus does not make predictions about the particle size distribution. To estimate whether terminator differences could be observable, we tried using a constant particle size as well as a size distribution from a 1D microphysics model [3]. In either case, one obtains a relatively small difference in transit depth between leading and trailing limb (Fig. 4). We note that the simulated spectral slope at short wavelengths is too flat to match the observational data. There could be multiple factors explaining why our model does not reproduce the slope of the data, including that a fully coupled three-dimensional microphysics and circulation model might be needed to reproduce the observational data. Furthermore, part of the slope could arise from unaccounted star spots. Another possibility is that mixing from small-scale turbulence not resolved by the GCM could be much more important than expected. In that case, the haze mass mixing ratio would decline more rapidly with increasing pressure, resulting in a steeper spectral slope.
References:
[1] Pont, F., Sing, D. K., Gibson, N. P., et al.: The prevalence of dust on the exoplanet HD 189733b from Hubble and Spitzer observations, Monthly Notices of the Royal Astronomical Society, 432, 4, 2917-2944, 2013.
[2] Sing, D. K., Fortney, J. J., Nikolov, N., et al.: A continuum from clear to cloudy hot-Jupiter exoplanets without primordial water depletion, Nature, 529, 7584, 59-62, 2016.
[3] Lavvas, P. and Koskinen, T.: Aerosol Properties of the Atmospheres of Extrasolar Giant Planets, The Astrophysical Journal, 847, 1, 32, 2017.
[4] Ohno, K. and Kawashima, Y.: Super-Rayleigh Slopes in Transmission Spectra of Exoplanets Generated by Photochemical Haze, The Astrophysical Journal Letters, 895, 2, L47, 2020.
[5] Parmentier, V., Showman, A. P. and Lian, Y.: 3D mixing in hot Jupiters atmospheres I . Application to the day / night cold trap in HD 209458b, Astronomy & Astrophysics, 558, A91, 2013.
[6] Lee, G., Dobbs-Dixon, I., Helling, Ch., et al.: Dynamic mineral clouds on HD 189733b. I. 3D RHD with kinetic, non-equilibrium cloud formation, Astronomy & Astrophysics, 594, A48, 2016.
[7] Lines, S., Mayne, N. J., Boutle, I. A., et al.: Simulating the cloudy atmospheres of HD 209458 b and HD 189733 b with the 3D Met Office Unified Model, Astronomy & Astrophysics, 615, A97, 2018.
[8] Line, M. and Parmentier, V.: The Influence of Nonuniform Cloud Cover on Transit Transmission Spectra, The Astrophysical Journal, 820, 1, 78, 2016.
[9] Kempton E. M. R., Bean J. L., Parmentier V.: An Observational Diagnostic for Distinguishing between Clouds and Haze in Hot Exoplanet Atmospheres, The Astrophysical Journal, 845, 2, L20, 2017.
[10] Adcroft A., Campin J.-M., Hill C., Marshall J.: Implementation of an Atmosphere Ocean General Circulation Model on the Expanded Spherical Cube, Monthly WeatherReview, 132, 2845, 2004.
How to cite: Steinrueck, M., Showman, A., Lavvas, P., Koskinen, T., Zhang, X., and Tan, X.: Three-dimensional Simulations of Photochemical Hazes in the Atmosphere of Hot Jupiter HD 189733b, Europlanet Science Congress 2020, online, 21 Sep–9 Oct 2020, EPSC2020-768, https://doi.org/10.5194/epsc2020-768, 2020.
Thanks to the advances in modern instrumentation we learned about many exoplanets that spawn a wide range of masses and composition. Studying their atmospheres provides insight into planetary diversity, origin, evolution, dynamics, and habitability. Present and future observing facilities will address these important topics in very detail by using more precise observations, high-resolution spectroscopy, improved analysis methods, etc. In this contribution we focus on the analysis of temperature and disequilibrium chemical processes in hot Jupiter atmospheres. In particular, we investigate the impact of photochemistry and vertical transport processes on mixing ratio profiles and on the simulated spectra of a hot Jupiters that orbits stars of various spectral types. We additionally address the impact of stellar activity that should be present in all stars with convective envelopes. Finally, we estimate the characterization of these processes using space and ground-based observations that will be carried out with near-future instruments and missions.
How to cite: Shulyak, D., Rengel, M., Lara, L., and Nemec, N.: Studying physics and chemistry in atmospheres of hot Jupiters from future ground-based and space facilities, Europlanet Science Congress 2020, online, 21 Sep–9 Oct 2020, EPSC2020-236, https://doi.org/10.5194/epsc2020-236, 2020.
Abstract
Direct-imaging observations of exoplanets in reflected starlight are expected to be available this decade. This will improve our knowledge about cold and temperate exoplanets and their atmospheres. Current theoretical efforts related to such planets aim to understand the effects that planet and atmospheric properties have on the spectra to be measured. This will help predict the science outcome of direct-imaging missions and identify the key needs for models used in the interpretarion of future measurements. In this work, we have investigated the information contained in reflected-light exoplanetary spectra and the role played by the planet radius in the atmospheric characterization.
Introduction
Long-period exoplanets are a population that remains substantially unexplored because of the biases introduced by the technology currently available. These planets have small transit probabilities due to their large orbital distances and hence the direct-imaging technique will be key to analyse their atmospheres. Space missions such as NGRST (formerly WFIRST), LUVOIR or HabEx will allow us to study this population of long-period exoplanets, providing a more complete picture of exoplanet diversity.
Model
We set up an atmospheric model with hydrogen and helium as the main components. We include methane, in a volume-mixing-ratio fCH4, as the only absorbing gaseous species. We add a cloud layer, described by its optical thickness (τc), its geometrical extension and the position of the cloud top. The aerosols are modelled by their single-scattering albedo and their effective radius, which determines the scattering phase function through Mie theory. This model is motivated by previous modelling of the atmospheres of Solar System gas giants. Apart from the six atmospheric parameters, we include the planet radius (Rp) as another model parameter. We apply our analysis to a particular target, Barnard's Star b candidate super-Earth[1], although our conclusions are generally planet-independent.
Retrieval
We built a grid of ~300,000 synthetic reflected-light spectra for a range of possible atmospheric configurations. The spectra were computed at phase angle α=0º (that is, with the exoplanet fully illuminated). The wavelength interval under study is 500-900 nm and the spectral resolution, R~125-225. The multiple-scattering radiative-transfer problem was solved with a previously validated code[2].
Observations were simulated by adding wavelength-independent noise at S/N=10. Three atmospheric configurations were considered to simulate observations and carry out retrievals: a cloud-free one (τc=0.05), one with a thin-cloud (τc=1.0) and one with a thick-cloud (τc=20.0). We developed an MCMC-based retrieval package achieving a continuous sampling of the parameter space by interpolating within the pre-computed grid of spectra.
The retrievals were carried out for cases where the planet radius was either known (and hence there were only 6 free parameters) or completely unconstrained (7 free parameters). We also analysed intermediate scenarios in which estimates of Rp with different uncertainties were assumed.
Results
The retrievals of atmospheric properties degrade as the uncertainties in the value of Rp increase. Indeed, the correlations between model parameters triggered by adding Rp as a free parameter make it challenging to distinguish between cloudy- and cloud-free atmospheres. Fig. 1 shows that, even if the atmosphere contains a thick-cloud, the evidence for the cloud disappears as the uncertainties in Rp grow. When the planet radius is a priori unconstrained, the retrieval of τc shows a nearly-flat posterior probability distribution. This indicates that the evidence is equal for both thick clouds or cloud-free atmospheres. On the other hand, if Rp is known we can generally distinguish between cloudy or cloud-free atmospheres in all of the scenarios analysed in this work. The retrieval results for other parameters such as the methane abundance also improve if the planet radius is known.
Fig. 1 also shows that a priori estimates on the value of Rp improve the retrievals. This result encourages the development of synergies between direct-imaging and other techniques in order to reduce the uncertainties in the mass and radius of long-period exoplanets.
Besides, we find that, if Rp is completely unconstrained, direct-imaging observations can constrain its value to within a factor of ~2 for all the cases explored. This might help start addressing the bulk composition of an exoplanet.
Several works have addressed the possible science return of future direct-imaging observations[3]-[5]. Since the exoplanets observed in direct-imaging will generally lack a measurement of Rp, we conclude that this parameter will play an important role in the retrievals and therefore should be included in this type of retrieval exercises.
References
[1] Ribas et al. (2018), Nature, 563, 365
[2] García Muñoz & Mills (2015), A&A, 573, A72
[3] Lupu et al. (2016), AJ, 152, 217
[4] Nayak et al. (2017), PASP, 129, 973
[5] Damiano & Hu (2019), AJ, 159, 175
How to cite: Carrión-González, Ó., García Muñoz, A., Cabrera, J., Csizmadia, S., Santos, N. C., and Rauer, H.: Directly imaged exoplanets in reflected starlight. The importance of knowing the planet radius, Europlanet Science Congress 2020, online, 21 Sep–9 Oct 2020, EPSC2020-674, https://doi.org/10.5194/epsc2020-674, 2020.