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Register a new userA SEA BASS on the exoplanet HD209458b
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Authors
Giuseppe Morello
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We present here the first application of Stellar and Exoplanetary Atmospheres Bayesian Analysis Simultaneous Spectroscopy (SEA BASS) on real datasets. SEA BASS is a scheme that enables the simultaneous derivation of four-coefficient stellar limb-darkening profiles, transit depths, and orbital parameters from exoplanetary transits at multiple wavelengths. It relies on the wavelength-independence of the system geometry and on the reduced limb-darkening effect in the infrared. This approach has been introduced by Morello et al. (2017) (without the SEA BASS acronym), who discuss several tests on synthetic datasets. Here, we (1) improve on the original algorithm by using multiple Spitzer/InfraRed Array Camera passbands and a more effective set of geometric parameters, (2) demonstrate its ability with Hubble Space Telescope/Space Telescope Imaging Spectrograph datasets, by (3) measuring the HD209458 stellar limb-darkening profile over multiple passbands in the 290-570 nm range with sufficient precision to rule out some theoretical models that have been adopted previously in theliterature, and (4) simultaneously extracting the transmission spectrum of the exoplanet atmosphere. The higher photometric precision of the next-generation instruments, such as those onboard the James Webb Space Telescope, will enable modeling the star-planet systems with unprecedented detail, and increase the importance of SEA BASS for avoiding the potential biases introduced by inaccurate stellar limb-darkening models.
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For extrasolar planets discovered using the radial velocity method, the spectral characterization of the host star leads to a mass-estimate of the star and subsequently of the orbiting planet. In contrast, if also the orbital velocity of the planet would be known, the masses of both star and planet could be determined directly using Newtons law of gravity, just as in the case of stellar double-line eclipsing binaries. Here we report on the detection of the orbital velocity of extrasolar planet HD209458b. High dispersion ground-based spectroscopy during a transit of this planet reveals absorption lines from carbon monoxide produced in the planet atmosphere, which shift significantly in wavelength due to the change in the radial component of the planet orbital velocity. These observations result in a mass determination of the star and planet of 1.00+-0.22 Msun and 0.64+-0.09 Mjup respectively. A ~2 km/sec blueshift of the carbon monoxide signal with respect to the systemic velocity of the host star suggests the presence of a strong wind flowing from the irradiated dayside to the non-irradiated nightside of the planet within the 0.01-0.1 mbar atmospheric pressure range probed by these observations. The strength of the carbon monoxide signal suggests a CO mixing ratio of 1-3x10-3 in this planets upper atmosphere.
A complete reassessment of the HST observations of the transits of the extrasolar planet HD209458b has provided a transmission spectrum of the atmosphere over a wide range of wavelengths. Analysis of the NaI absorption line profile has already shown that the sodium abundance has to drop by at least a factor of ten above a critical altitude. Here we analyze the profile in the deep core of the NaI doublet line from HST and high-resolution ground-based spectra to further constrain the vertical structure of the HD209458b atmosphere. With a wavelength-dependent cross section that spans more than 5 orders of magnitude, we use the absorption signature of the NaI doublet as an atmospheric probe. The NaI transmission features are shown to sample the atmosphere of HD209458b over an altitude range of more than 6500km, corresponding to a pressure range of 14 scale heights spanning 1 millibar to 1e-9 bar pressures. By comparing the observations with a multi-layer model in which temperature is a free parameter at the resolution of the atmospheric scale height, we constrain the temperature vertical profile and variations in the Na abundance in the upper part of the atmosphere of HD209458b. We find a rise in temperature above the drop in sodium abundance at the 3mbar level. We also identify an isothermal atmospheric layer at 1500+/-100K spanning almost 6 scale heights in altitude, from 1e-5 to 1e-7 bar. Above this layer, the temperature rises again to 2500(+1500/-1000)K at 1e-9 bar, indicating the presence of a thermosphere. The resulting temperature-pressure (T-P) profile agrees with the Na condensation scenario at the 3 mbar level, with a possible signature of sodium ionization at higher altitudes, near the 3e-5 bar level. Our T-P profile is found to be in good agreement with the profiles obtained with aeronomical models including hydrodynamic escape.
Transit observations in the MgI line of HD209458b revealed signatures of neutral magnesium escaping the upper atmosphere of the planet, while no atmospheric absorption was found in the MgII doublet. Here we present a 3D particle model of the dynamics of neutral and ionized magnesium populations, coupled with an analytical modeling of the atmosphere below the exobase. Theoretical MgI absorption line profiles are directly compared with the absorption observed in the blue wing of the line during the planet transit. Observations are well-fitted with an escape rate of neutral magnesium in the range 2x10^7-3.4x10^7 g/s, an exobase close to the Roche lobe (Rexo in the range 2.1-4.3 Rp, where Rp is the planet radius) and a planetary wind velocity at the exobase vpl=25km/s. The observed velocities of the planet-escaping magnesium up to -60km/s are well explained by radiation pressure acceleration, provided that UV-photoionization is compensated for by electron recombination up to about 13Rp. If the exobase properties are constrained to values given by theoretical models of the deeper atmosphere (Rexo=2Rp and vpl=10km/s), the best fit to the observations is found at a similar electron density and escape rate within 2 sigma. In all cases, the mean temperature of the atmosphere below the exobase must be higher than about 6100 K. Simulations predict a redward expansion of the absorption profile from the beginning to the end of the transit. The spatial and spectral structure of the extended atmosphere is the result of complex interactions between radiation pressure, planetary gravity, and self-shielding, and can be probed through the analysis of transit absorption profiles in the MgI line.
We present a new approach for simulating the atmospheric dynamics of the close-in giant planet HD209458b that allows for the decoupling of radiative and thermal energies, direct stellar heating of the interior, and the solution of the full 3D Navier Stokes equations. Simulations reveal two distinct temperature
We report the detection of an atmosphere on a rocky exoplanet, GJ 1132 b, which is similar to Earth in terms of size and density. The atmospheric transmission spectrum was detected using Hubble WFC3 measurements and shows spectral signatures of aerosol scattering, HCN, and CH$_{4}$ in a low mean molecular weight atmosphere. We model the atmospheric loss process and conclude that GJ 1132 b likely lost the original H/He envelope, suggesting that the atmosphere that we detect has been reestablished. We explore the possibility of H$_{2}$ mantle degassing, previously identified as a possibility for this planet by theoretical studies, and find that outgassing from ultrareduced magma could produce the observed atmosphere. In this way we use the observed exoplanet transmission spectrum to gain insights into magma composition for a terrestrial planet. The detection of an atmosphere on this rocky planet raises the possibility that the numerous powerfully irradiated Super-Earth planets, believed to be the evaporated cores of Sub-Neptunes, may, under favorable circumstances, host detectable atmospheres.
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