Exoplanet transits enable high-resolution spectroscopy across spatially resolved stellar surfaces

Exoplanet transits enable high-resolution spectroscopy across spatially resolved stellar surfaces

Authors:

Dravins et al

Abstract:

Observations of stellar surfaces - except for the Sun - are hampered by their tiny angular extent, while observed spectral lines are smeared by averaging over the stellar surface, and by stellar rotation. Exoplanet transits can be used to analyze stellar atmospheric structure, yielding high-resolution spectra across spatially highly resolved stellar surfaces, free from effects of spatial smearing and the rotational wavelength broadening present in full-disk spectra. During a transit, stellar surface portions successively become hidden, and differential spectroscopy between various transit phases provides spectra of those surface segments then hidden behind the planet. The small area subtended by even a large planet (about 1% of a main-sequence star) offers high spatial resolution but demands very precise observations. We demonstrate the reconstruction of photospheric FeI line profiles at a spectral resolution R=80,000 across the surface of the solar-type star HD209458. Any detailed understanding of stellar atmospheres requires modeling with 3-dimensional hydrodynamics. The properties predicted by such models are mapped onto the precise spectral-line shapes, asymmetries and wavelength shifts, and their variation from the center to the limb across any stellar disk. This method provides a tool for testing and verifying such models. The method will soon become applicable to more diverse types of stars, thanks to new spectrometers on very large telescopes, and since ongoing photometric searches are expected to discover additional bright host stars of transiting exoplanets.

Hot Jupiter HD 209458b's Atmosphere Does NOT Have a Thermal Inversion Layer

New Analysis Indicates No Thermal Inversion in the Atmosphere of HD 209458b

Authors:

Diamond-Lowe et al

Abstract:

An important focus of exoplanet research is the determination of the atmospheric temperature structure of strongly irradiated gas giant planets, or hot Jupiters. HD 209458b is the prototypical exoplanet for atmospheric thermal inversions, but this assertion does not take into account recently obtained data or newer data reduction techniques. We re-examine this claim by investigating all publicly available Spitzer Space Telescope secondary-eclipse photometric data of HD 209458b and performing a self-consistent analysis. We employ data reduction techniques that minimize stellar centroid variations, apply sophisticated models to known Spitzer systematics, and account for time-correlated noise in the data. We derive new secondary-eclipse depths of 0.119 +/- 0.007%, 0.123 +/- 0.006%, 0.134 +/- 0.035%, and 0.215 +/- 0.008% in the 3.6, 4.5, 5.8, and 8.0 micron bandpasses, respectively. We feed these results into a Bayesian atmospheric retrieval analysis and determine that it is unnecessary to invoke a thermal inversion to explain our secondary-eclipse depths. The data are well-fitted by a temperature model that decreases monotonically between pressure levels of 1 and 0.01 bars. We conclude that there is no evidence for a thermal inversion in the atmosphere of HD 209458b.

Examining HD 209458b Hints at Overestimate of H2 Escape From Exoplanetary Atmospheres

Heating efficiency in hydrogen-dominated upper atmospheres

Authors:

Shemtovich et al

Abstract:

Context.

The heating efficiency is defined as the ratio of the net local gas-heating rate to the rate of stellar radiative energy absorption. It plays an important role in thermal-escape processes from the upper atmospheres of planets that are exposed to stellar soft X-rays and extreme ultraviolet radiation (XUV).

Aims.

We model the thermal-escape-related heating efficiency of the stellar XUV radiation in the hydrogen-dominated upper atmosphere of the extrasolar gas giant HD 209458b. The model result is then compared with previous thermal-hydrogen-escape studies which assumed heating efficiency values between 10-100%.

Methods.

The photolytic and electron impact processes in the thermosphere were studied by solving the kinetic Boltzmann equation and applying a Direct Simulation Monte Carlo model. We calculated the energy deposition rates of the stellar XUV flux and that of the accompanying primary photoelectrons that are caused by electron impact processes in the H2 to H transition region in the upper atmosphere.

Results.

The heating by XUV radiation of hydrogen-dominated upper atmospheres does not reach higher than 20% above the main thermosphere altitude, if the participation of photoelectron impact processes is included.

Conclusions.

Hydrogen-escape studies from exoplanets that assume heating efficiency values that are greater than or equal to 20 % probably overestimate the thermal escape or mass-loss rates, while those who assumed values that are less than 20% probably produce more realistic atmospheric-escape rates.