Limits on the Post-eclipse Emission Spectrum of HD 80606 b From High-Resolution Spectroscop

We present Keck/NIRSPEC $K$-band observations of HD 80606 b, one of the most eccentric known exoplanets. HD 80606 b was observed after secondary eclipse, close to periastron, when the planet passes within 0.03 AU of HD 80606 and the rapid heating of the atmosphere may lead to extreme chemical changes and a temporary thermal inversion. The rapid change in the planetary radial velocity near periastron is sufficient to enable high-resolution cross-correlation spectroscopy (HRCCS) analysis, which produces a tentative detection ($\rm SNR\sim4$) of HD 80606 b. Injection-recovery tests appear to reject strong thermal inversions near periastron, consistent with recent results from JWST. We also perform atmospheric retrievals with free parameters for the Pressure-Temperature ($P-T$) profile and with a profile matched to the JWST results, which suggest the presence of absorption features from CH$_4$ and CO. While HD 80606 b is not definitively detected in these data, these results are consistent with JWST observations, which found the post-eclipse atmosphere of HD 80606 b shows weak absorption features from these species. Future observations with higher spectral resolution and/or wider wavelength coverage are needed for a confident atmospheric detection of HD 80606 b via high-resolution spectroscopy alone, but such observations are a challenge to schedule due to the 111-day orbital period.

💡 Research Summary

The authors present a high‑resolution (R ≈ 30,000) K‑band spectroscopic study of the highly eccentric hot‑Jupiter HD 80606 b using Keck II/NIRSPEC, targeting the planet’s emission shortly after secondary eclipse, when it is near periastron (≈0.03 AU from its G5V host). The rapid change in the planet’s radial velocity (≈120 km s⁻¹ over a few hours) makes high‑resolution cross‑correlation spectroscopy (HRCCS) feasible despite the modest signal‑to‑noise ratio (SNR).

Observations were carried out on 2024‑01‑21, covering the post‑eclipse phase from 10:45 UT to 14:20 UT. The authors discarded the pre‑eclipse data (high airmass, poor weather) and combined consecutive 60‑second exposures into 90 co‑added frames for analysis. Data reduction employed a modified KPIC pipeline: trace shifts along the slit were measured and corrected, a Gaussian line‑spread function (σ = 1.3 pixels) was adopted to account for the broadened instrumental profile, and wavelength calibration combined M‑type giant stellar lines with a PHOENIX stellar model and a telluric model from the Planetary Spectrum Generator. The final wavelength solution achieved a radial‑velocity precision of 0.4–1.0 km s⁻¹.

Systematics were removed by normalizing each spectrum, masking low‑transmission and high‑variance channels, applying a 6‑σ median‑absolute‑deviation clip, and subtracting 4–8 principal components (PCs) per order. The removed PCs were later added back into the forward model to preserve any planet‑induced distortions.

Cross‑correlation with model templates (CO, H₂O, CH₄) yielded a tentative detection of the planet at SNR ≈ 4, centered near the expected orbital velocity (Kₚ ≈ 124 km s⁻¹) and systemic offset (Δvₛᵧₛ ≈ 0 km s⁻¹). The signal is not statistically robust, and the authors treat it as a possible detection pending higher‑quality data.

Atmospheric retrievals were performed using petitRADTRANS coupled with a nested‑sampling framework (PyMultiNest). Two retrieval setups were explored: (1) a free pressure‑temperature (P‑T) profile with free volume mixing ratios for H₂O, CO, CH₄, and H₂; (2) a P‑T profile fixed to the one derived from recent JWST observations, allowing only the chemical abundances to vary. Both approaches favor the presence of CO and CH₄ absorption, with retrieved log‑mixing ratios of roughly –1.8 ± 1.7 for CO and –4.8 ± 3.3 for CH₄. The infrared opacity parameters (κ, γ) converge toward values that do not support a strong thermal inversion, consistent with JWST MIRI LRS results that indicate only a weak or absent inversion in the post‑eclipse atmosphere.

Injection‑recovery tests demonstrate that a model with a strong thermal inversion would be readily recovered if present, but such a model fails to match the observed data, reinforcing the conclusion that HD 80606 b lacks a pronounced inversion near periastron.

The retrieval also yields modest constraints on the planet’s rotational broadening (v_rot ≈ 1.5–7.5 km s⁻¹) and a systemic velocity offset of –11 ± 16 km s⁻¹, slightly larger than the typical 2–5 km s⁻¹ blueshift seen in tidally locked hot Jupiters, possibly reflecting asymmetric winds driven by the extreme, rapidly changing irradiation at periastron.

In summary, the study shows that high‑resolution spectroscopy can probe the rapidly moving atmosphere of an eccentric hot Jupiter, providing tentative evidence for CO and CH₄ absorption and ruling out strong thermal inversions. However, the modest SNR limits a definitive detection. The authors recommend future observations with higher spectral resolution (R > 60,000), broader wavelength coverage (1–5 µm), and longer, uninterrupted time series to overcome the scheduling challenges posed by the 111‑day orbital period. Such data would enable a confident, high‑resolution spectroscopic characterization of HD 80606 b’s atmosphere, complementing JWST’s low‑resolution but broader‑band observations.