We describe a detailed study on the properties of alkali atoms in extrasolar giant planets, and specifically focus on their role in generating the atmospheric free electron densities, as well as their impact on the transit depth observations. We focus our study on the case of HD 209458b, and we show that photoionization produces a large electron density in the middle atmosphere that is about two orders of magnitude larger than the density anticipated from thermal ionization. Our purely photochemical calculations, though, result in a much larger transit depth for K than observed for this planet. This result does not change even if the roles of molecular chemistry and excited state chemistry are considered for the alkali atoms. In contrast, the model results for the case of exoplanet XO-2b are in good agreement with the available observations. Given these results we discuss other possible scenarios, such as changes in the elemental abundances, changes in the temperature profiles, and the possible presence of clouds, which could potentially explain the observed HD 209458b alkali properties. We find that most of these scenarios cannot explain the observations, with the exception of a heterogeneous source (i.e., clouds or aerosols) under specific conditions, but wemore » also note the discrepancies among the available observations.« less

https://iopscience.iop.org/article/10.1088/0004-637X/796/1/15/pdf

Electron densities and alkali atoms in exoplanet atmospheres

This paper presents the experimental measurement and computational model of sodium plasma decay processes in mixture of sodium and argon by using radar resonance-enhanced multiphoton ionization (REMPI), coherent microwave Rayleigh scattering of REMPI. A single laser beam resonantly ionizes the sodium atoms by means of 2+1 REMPI process. The laser beam can only generate the ionization of the sodium atoms and have negligible ionization of argon. Coherent microwave scattering in situ measures the total electron number in the laser-induced plasma. Since the sodium ions decay by recombination with electrons, microwave scattering directly measures the plasma decay processes of the sodium ions. A theoretical plasma dynamic model, including REMPI of the sodium and electron avalanche ionization (EAI) of sodium and argon in the gas mixture, has been developed. It confirms that the EAI of argon is several orders of magnitude lower than the REMPI of sodium. The theoretical prediction made for the plasma decay process of sodium plasma in the mixture matches the experimental measurement.

Measurement of plasma decay processes in mixture of sodium and argon by coherent microwave scattering

Exoplanets orbiting close to their host star are expected to support a large ionosphere, which extends to larger pressures than witnessed in our Solar System. These ionospheres can be investigated with ground-based transit observations of the optical signatures of alkali metals, which are the source of the ions. Here, we present new observations and analyses of optical transit spectra that cover the Na doublet in the atmosphere of the exoplanet XO-2 b. To assess the consistency of our results, observations were obtained from two separate platforms: Gemini/GMOS and Mayall/KOSMOS. To mitigate the systematic errors, we chose XO-2, because it has a binary companion of the same brightness and stellar type, which provides an ideal reference star to model Earth's atmospheric effects. We find that interpretation of the data is highly sensitive to time-varying translations along the detector, which change according to wavelength and differ between the target and reference star. It was necessary to employ a time-dependent cross-correlation to align our wavelength bins and correct for atmospheric differential refraction. This approach allows us to resolve the wings of the Na line across 5 wavelength bins at a resolution of $\sim$1.6nm and limit the abundance of Na. We obtain consistent results from each telescope with a Na amplitude of 521$\pm$161 ppm and 403$\pm$186 ppm for GMOS and KOSMOS respectively. The results are analyzed with a radiative transfer model that includes the effects of ionization. The data are consistent with a clear atmosphere between $\sim$1--100 mbar which establish a lower limit on Na at 0.4$^{+2}_{-0.3}$ ppm ([Na/H]=-0.64$^{+0.78}_{-0.6}$), consistent with solar. However, we can not rule out the presence of clouds at $\sim$10 mbar which allow for higher Na abundances which would be consistent with stellar metallicity measured for the host star ([Na/H]=0.485$\pm$0.043).

https://iopscience.iop.org/article/10.3847/1538-3881/aaf1ae/pdf

Ground-based spectroscopy of the exoplanet XO-2B using a systematic wavelength calibration

Ground-based Spectroscopy of the Exoplanet XO-2b using a Systematic Wavelength Calibration.

This paper presents the current experimental and computational efforts of measuring sodium plasma decay processes in mixture of sodium and argon by using Radar REMPI, coherent microwave Rayleigh scattering of Resonance Enhanced Multi-Photon Ionization. A single laser beam resonantly ionizes the sodium atoms by means of 2+1 REMPI process which can only generate ionization of the sodium atoms and have negligible ionization of the argon. Coherent microwave scattering in-situ measures the total electron number in the laser-induced plasma. Since the sodium ions decays by recombination with electrons, microwave scattering directly measures the plasma decay processes of the sodium ions. A theoretical plasma dynamic model, including REMPI of the sodium and Electron Avalanche Ionization (EAI) of sodium and argon in the gas mixture has been developed. It confirms that the EAI of argon is several-order-magnitude lower than the REMPI of sodium. The theoretical prediction made for the plasma decay process of sodium plasma in the mixture matches experimental measurement.

http://enu.kz/repository/2009/AIAA-2009-4301.pdf

Measurement of Plasma Decay Processes in Mixture of Sodium and Argon by Radar REMPI

The ionization kinetics of sodium diluted in argon is studied in a shock tube, in which the test gas mixture is ionized by a reflected shock wave and subsequently quenched by a strong rarefaction wave. A Langmuir electrostatic probe is used to monitor the variation of the ion number density at the reflection shock wave region. The working state of the probe is in the near free fall region and a correction for reduction of the probe current due to elastic scattering in the probe sheath is introduced. At the temperature range of 800 to 2600 K and in the ambience of argon gas, the three-body recombination rate coefficient of the sodium ion with electron is determined: 3.43×10-14T-3.77cm6s-1.

https://iopscience.iop.org/article/10.1088/0256-307X/18/2/346/pdf

HE Yu-Zhong

Papers

Shock Tube Studies on Recombination Kinetics of Sodium Ion with Electron