[Paper Review] Investigation of the upper atmosphere in ultra-hot Jupiter WASP-76 b with high-resolution spectroscopy

Kiyoe Kawauchi, Norio Narita+2 more|arXiv (Cornell University)|Dec 5, 2021

Stellar, planetary, and galactic studiesPhysics and Astronomy61 references12 citations

TL;DR

This study presents high-resolution transmission spectroscopy of the ultra-hot Jupiter WASP-76b using Subaru/HDS, detecting Na D absorption lines with enhanced breadth and depth. It reveals a thermospheric temperature of 3700–4200 K, significantly exceeding the equilibrium temperature of ~2160 K, indicating atmospheric heating due to stellar X-ray and EUV irradiation.

ABSTRACT

Alkali metal lines are one of the most important key opacity sources for understanding exoplanetary atmospheres because the Na I resonance doublets are thought to be the cause of low albedo, as the alkali metal's wide line wings absorb almost all of the incoming stellar irradiation. High-resolution transmission spectroscopy of Na absorption lines can be used to investigate the temperature of the thermosphere of hot Jupiters, which is increased by stellar X-ray and EUV irradiation. We applied high-resolution transmission spectroscopy to the ultra-hot Jupiter WASP-76 b with the High Dispersion Spectrograph (HDS) on the Subaru 8.2 m telescope. We report the detection of strong Na D excess absorption with line contrasts of 0.42 $\pm$ 0.03 % (D1 at 5895.92 {A}) and 0.38 $\pm$ 0.04 % (D2 at 5889.95 {A}), FWHMs of 1.63 $\pm$ 0.13 {A} (D1) and 1.87 $\pm$ 0.22 {A} (D2), and EWs of (7.29 $\pm$ 1.43) $ imes$ 10$^{-3}$ {A} (D1) and (7.56 $\pm$ 2.38) $ imes$ 10$^{-3}$ {A} (D2). These results show that the Na D absorption lines are shallower and broader than those in previous work, whereas the absorption signals over the same passband are consistent with those in previous work. We derive the best-fitted isothermal temperature of 3700 K (without rotation) and 4200 K (with rotation). These results suggest the possibility of the existence of a thermosphere because the derived atmospheric temperature is higher than the equilibrium temperature ($\sim$ 2160 K).

Motivation & Objective

• To investigate the upper atmospheric structure of the ultra-hot Jupiter WASP-76b using high-resolution transmission spectroscopy.
• To determine the temperature and dynamics of the thermosphere by analyzing Na D line profiles.
• To resolve discrepancies in prior high-resolution Na absorption measurements of WASP-76b by applying improved data reduction and modeling.
• To assess the role of atmospheric escape and wind patterns through line width and asymmetry analysis.
• To compare observed transmission spectra with theoretical models including thermal ionization and rotational effects.

Proposed method

• Observed WASP-76b during transit using the High Dispersion Spectrograph (HDS) on the Subaru 8.2 m telescope with a spectral resolution of ~90,000.
• Applied a custom data reduction pipeline to remove telluric, stellar, and instrumental features, correcting for the planet's radial velocity.
• Fitted the Na D line profiles with Gaussian and theoretical transmission models, including isothermal and radiative equilibrium assumptions.
• Incorporated thermal ionization of sodium and rotational effects in theoretical models to improve spectral fitting.
• Used Markov Chain Monte Carlo (MCMC) methods with the emcee package to estimate uncertainties and perform model comparison.
• Compared observed line contrasts, FWHM, and equivalent widths with low-resolution data and previous high-resolution studies.

Experimental results

Research questions

• RQ1 What is the precise line contrast, FWHM, and equivalent width of Na D absorption in WASP-76b’s atmosphere using high-resolution spectroscopy?
• RQ2 How do the derived Na D line parameters compare with previous measurements from HARPS and ESPRESSO?
• RQ3 What is the inferred thermospheric temperature of WASP-76b, and does it exceed the equilibrium temperature?
• RQ4 What atmospheric dynamics (e.g., winds, rotation, escape) are indicated by the broadened and asymmetric Na D line profiles?
• RQ5 How well do theoretical models including thermal ionization and rotation fit the observed transmission spectra?

Key findings

• The Na D1 line shows a contrast of 0.42 ± 0.03%, FWHM of 1.63 ± 0.13 Å, and equivalent width of (7.29 ± 1.43) × 10⁻³ Å.
• The Na D2 line exhibits a contrast of 0.38 ± 0.04%, FWHM of 1.87 ± 0.22 Å, and equivalent width of (7.56 ± 2.38) × 10⁻³ Å.
• The Na D line profiles are broader (FWHM ~80.7 ± 7.9 km/s in velocity space) than in previous studies, suggesting additional atmospheric winds beyond rotation.
• The best-fitting isothermal model yields a temperature of 3700 K without rotation and 4200 K with rotation, both exceeding the equilibrium temperature of ~2160 K.
• The observed line asymmetry and blue shift support day-night atmospheric chemistry, consistent with iron condensation on the nightside.
• Model comparison shows that the isothermal model with rotation and thermal ionization provides the best fit, with reduced χ² values of 0.38 (Fadd) and 0.48 (EMC).