Aerosol Constraints on the Atmosphere of the Hot Saturn-mass planet WASP-49b
Authors:
Cubillos et al
Abstract:
The strong, nearly wavelength-independent absorption cross section of aerosols produces featureless exoplanet transmission spectra, limiting our ability to characterize their atmospheres. Here we show that even in the presence of featureless spectra, we can still characterize certain atmospheric properties. Specifically, we constrain the upper and lower pressure boundaries of aerosol layers, and present plausible composition candidates. We study the case of the bloated Saturn-mass planet WASP-49b, where near-infrared observations reveal a flat transmission spectrum between 0.7 and 1.0 {\microns}. First, we use a hydrodynamic upper-atmosphere code to estimate the pressure reached by the ionizing stellar high-energy photons at 10−8 bar, setting the upper pressure boundary where aerosols could exist. Then, we combine HELIOS and Pyrat Bay radiative-transfer models to constrain the temperature and photospheric pressure of atmospheric aerosols, in a Bayesian framework. For WASP-49b, we constrain the transmission photosphere (hence, the aerosol deck boundaries) to pressures above 10−5 bar (100× solar metallicity), 10−4 bar (solar), and 10−3 bar (0.1× solar) as lower boundary, and below 10−7 bar as upper boundary. Lastly, we compare condensation curves of aerosol compounds with the planet's pressure-temperature profile to identify plausible condensates responsible for the absorption. Under these circumstances, we find as candidates: Na2S (at 100× solar metallicity); Cr and MnS (at solar and 0.1× solar); and forsterite, enstatite, and alabandite (at 0.1× solar).
FORS2 observes a multi-epoch transmission spectrum of the hot Saturn-mass exoplanet WASP-49b
Authors:
Lendl et al
Abstract:
Context:
Transmission spectroscopy has proven to be a useful tool for the study of exoplanet atmospheres, and has lead to the detection of a small number of elements and molecules (Na, K, H2O), but also revealed that many planets show flat transmission spectra consistent with the presence of opaque high-altitude hazes or clouds.
Aims:
We apply this technique to the MP=0.38Mjup, Rp=1.12Rjup, P=2.78d planet WASP-49b, aiming to characterize its transmission spectrum between 0.73 and 1 μm and search for the features of K and H2O.
Methods:
Three transits of WASP-49b have been observed with the FORS2 instrument installed at the VLT/UT1 telescope at the ESO Paranal site. We used FORS2's MXU mode with grism GRIS_600z, producing simultaneous multi-wavelength transit lightcurves throughout the i' and z' bands. We combined these data with independent broadband photometry from the Euler and TRAPPIST telescopes to obtain a good measurement of the transit shape. Strong correlated noise structures are present in the FORS2 lightcurves due to rotating flat field structures that are introduced by inhomogeneities of the linear atmospheric dispersion corrector's transparency. We account for these structures by constructing common noise models from the residuals of lightcurves bearing the same noise structures, and use these together with simple parametric models for the inference of the transmission spectrum.
Results:
We present three independent transmission spectra of WASP-49b between 0.73 and 1.02 μm, as well as a transmission spectrum between 0.65 and 1.02 μm from the combined analysis of FORS2 and broadband data. The results obtained from the three individual epochs agree well. The transmission spectrum of WASP-49b is best fit by atmospheric models containing a cloud deck at pressure levels of 1 mbar or less.
