The ground-based H, K, and L-band absolute emission spectra of HD 209458b

TL;DR

This study presents the first absolute L-band emission spectra of exoplanet HD 209458b using ground-based telescopes, finding no evidence of non-LTE CH4 fluorescence as previously observed in similar exoplanets.

Contribution

It demonstrates high-precision ground-based near-infrared emission spectroscopy of exoplanets and compares results with space-based observations, challenging prior fluorescence hypotheses.

Findings

No bright 3.3 μm fluorescence feature detected in HD 209458b.

Ground-based measurements agree with space-based data within 1σ.

Techniques significantly reduce flux variance, enabling precise exoplanet emission measurements.

Abstract

Here we explore the capabilities of NASA's 3.0 meter Infrared Telescope Facility (IRTF) and SpeX spectrometer and the 5.08 meter Hale telescope with the TripleSpec spectrometer with near-infrared H, K, and L-band measurements of HD 209458b's secondary eclipse. Our IRTF/SpeX data are the first absolute L-band spectroscopic emission measurements of any exoplanet other than the hot Jupiter HD 189733b. Previous measurements of HD 189733b's L-band indicate bright emission hypothesized to result from non-LTE CH$_{4}$ $\nu_{3}$ fluorescence. We do not detect a similar bright 3.3 $\mu$m feature to ~3$\sigma$, suggesting that fluorescence does not need to be invoked to explain HD 209458b's L-band measurements. The validity of our observation and reduction techniques, which decrease the flux variance by up to 2.8 orders of magnitude, is reinforced by 1$\sigma$ agreement with existent $Hubble$/NICMOS and $Spitzer$/IRAC1 observations which overlap the H, K, and L-bands, suggesting that both IRTF/SpeX and Palomar/TripleSpec can measure an exoplanet's emission with high precision.