Searching for Far-Ultraviolet Auroral/Dayglow Emission from HD209458b

TL;DR

This study used HST ultraviolet spectra to search for auroral emissions from exoplanet HD209458b, finding no definitive signals but identifying a potential H2 emission feature and setting upper limits on atmospheric emissions.

Contribution

First to analyze far-ultraviolet spectra of HD209458b for auroral emissions, providing constraints on atmospheric composition and magnetic field strength.

Findings

No unambiguous atomic or molecular emission features detected.

A statistically significant feature at 1582 A possibly from H2 emission.

Constraints on neutral hydrogen column density in the planetary atmosphere.

Abstract

We present recent observations from the HST-Cosmic Origins Spectrograph aimed at characterizing the auroral emission from the extrasolar planet HD209458b. We obtained medium-resolution (R~18-20,000) far-ultraviolet (1150-1700A) spectra at both the Phase 0.25 and Phase 0.75 quadrature positions as well as a stellar baseline measurement at secondary eclipse. This analysis includes a catalog of stellar emission lines and a star-subtracted spectrum of the planet. We present an emission model for planetary H2 emission, and compare this model to the planetary spectrum. No unambiguously identifiable atomic or molecular features are detected, and upper limits are presented for auroral/dayglow line strengths. An orbital velocity cross-correlation analysis finds a statistically significant (3.8 sigma) feature at +15 (+/- 20) km/s in the rest frame of the planet, at 1582 A. This feature is consistent with emission from H2 B-X (2-9) P(4) (lambda_{rest} = 1581.11 A), however the physical mechanism required to excite this transition is unclear. We compare limits on relative line strengths seen in the exoplanet spectrum with models of ultraviolet fluorescence to constrain the atmospheric column density of neutral hydrogen between the star and the planetary surface. These results support models of short period extrasolar giant planets with weak magnetic fields and extended atomic atmospheres.