HST/STIS Optical Transit Transmission Spectra of the hot-Jupiter HD209458b

TL;DR

This study presents high-resolution optical transmission spectra of HD209458b, revealing detailed atmospheric absorption features, including sodium and an additional absorber, while assessing limb-darkening corrections and model limitations.

Contribution

First comprehensive pixel-by-pixel limb-darkening correction applied to high-resolution spectra of HD209458b, revealing detailed atmospheric absorption features and assessing limb-darkening models.

Findings

Na I absorption reaches ~0.11% in a 4.4 Å band

Detection of an additional absorber at ~6,250 Å

Atlas limb-darkening models slightly overestimate limb-darkening

Abstract

We present the transmission spectra of the hot-Jupiter HD209458b taken with the Space Telescope Imaging Spectrograph aboard the Hubble Space Telescope. Our analysis combines data at two resolutions and applies a complete pixel-by-pixel limb-darkening correction to fully reveal the spectral line shapes of atmospheric absorption features. Terrestrial-based Na I and H I contamination are identified which mask the strong exoplanetary absorption signature in the Na core, which we find reaches total absorption levels of ~0.11% in a 4.4 Ang band. The Na spectral line profile is characterized by a wide absorption profile at the lowest absorption depths, and a sharp transition to a narrow absorption profile at higher absorption values. The transmission spectra also shows the presence of an additional absorber at ~6,250 Ang, observed at both medium and low resolutions. We performed various limb-darkening tests, including using high precision limb-darkening measurements of the sun to characterize a general trend of Atlas models to slightly overestimate the amount of limb-darkening at all wavelengths, likely due to the limitations of the model's one-dimensional nature. We conclude that, despite these limitations, Atlas models can still successfully model limb-darkening in high signal-to-noise transits of solar-type stars, like HD209458, to a high level of precision over the entire optical regime (3,000-10,000 Ang) at transit phases between 2nd and 3rd contact.