[Paper Review] Hot Exoplanet Atmospheres Resolved with Transit Spectroscopy (HEARTS) - II. A broadened sodium feature on the ultra-hot giant WASP-76b

J. V. Seidel, D. Ehrenreich+19 more|UvA-DARE (University of Amsterdam)|Jan 30, 2019

Stellar, planetary, and galactic studies50 references35 citations

TL;DR

The HEARTS survey detects neutral sodium in WASP-76b via transit spectroscopy with HARPS, finding a significantly broadened Na I line that may trace upper-atmosphere dynamics such as super-rotation. The signal is repeatable across three transits with high statistical significance.

ABSTRACT

High-resolution optical spectroscopy is a powerful tool to characterise exoplanetary atmospheres from the ground. The sodium D lines, with their large cross sections, are especially suited to study the upper layers of atmospheres in this context. We report on the results from HEARTS, a spectroscopic survey of exoplanet atmospheres, performing a comparative study of hot gas giants to determine the effects of stellar irradiation. In this second installation of the series, we highlight the detection of neutral sodium on the ultra-hot giant WASP-76b. We observed three transits of the planet using the HARPS high-resolution spectrograph at the ESO 3.6m telescope and collected 175 spectra of WASP-76. We repeatedly detect the absorption signature of neutral sodium in the planet atmosphere ($0.371\pm0.034\%$; $10.75 σ$ in a $0.75$ Å passband). The sodium lines have a Gaussian profile with full width at half maximum (FWHM) of $27.6\pm2.8$ km s$^{-1}$. This is significantly broader than the line spread function of HARPS ($2.7$ km s$^{-1}$). We surmise that the observed broadening could trace the super-rotation in the upper atmosphere of this ultra-hot gas giant.

Motivation & Objective

• Study the upper atmospheric properties of hot gas giants through high-resolution transmission spectroscopy, focusing on sodium as a key diagnostic

Proposed method

• Observe three transits of WASP-76b with HARPS at the ESO 3.6m telescope
• Concentrate on HARPS spectral order 56 containing the Na I D lines (5890-5896 Å region)
• Apply telluric correction with molecfit to remove atmospheric lines
• Perform simultaneous EulerCam photometry to constrain transit parameters and baselines
• Construct transmission spectra by comparing in-transit and out-of-transit spectra in the planetary rest frame
• Fit Gaussian profiles to the Na I line cores to derive line depths and FWHMs
• Assess repeatability across nights and evaluate potential CLV/RM effects
• Account for planetary radial velocity shifts during transit when computing the transmission spectrum

Experimental results

Research questions

• RQ1 Is neutral sodium detected in the atmosphere of WASP-76b during transit?
• RQ2 What is the line profile (depth and width) of the Na I doublet in WASP-76b, and what does it imply about atmospheric dynamics?
• RQ3 Is the sodium signal repeatable across multiple transits, and how do observational systematics affect it?
• RQ4 Do center-to-limb variation (CLV) or Rossiter-McLaughlin effects significantly bias the transmission spectrum for this system?
• RQ5 What is the atmospheric height corresponding to the detected sodium absorption?

Key findings

• Neutral sodium is repeatedly detected in WASP-76b’s atmosphere with absorption depths of 0.373% and 0.508% for the Na I D2 and D1 lines in a 0.75 Å passband, respectively (0.371±0.034% and 0.508±0.083% in a 0.75 Å passband).
• The Na I lines have a Gaussian profile with FWHM of 27.6±2.8 km/s, significantly broader than HARPS’ instrumental line spread function of 2.7 km/s.
• A strong core-centered sodium signal is observed in passbands as narrow as 0.75 Å, with a detection level of 10.75σ in a 2×0.75 Å central passband.
• The transmission signal is repeatable across the three nights, with in-night depths consistent within 1σ for the 2×0.75 Å band.
• Assessed atmospheric height corresponding to the strongest core absorption: 28,800±2,600 km (0.19±0.02 Rp).
• The broadening is suggested to trace super-rotation in the upper atmosphere of WASP-76b.