Detection of carbon monoxide in the high-resolution day-side spectrum of the exoplanet HD 189733b

Abstract: [Abridged] After many attempts over more than a decade, high-resolution spectroscopy has recently delivered its first detections of molecular absorption in exoplanet atmospheres, both in transmission and thermal emission spectra. Targeting the combined signal from individual lines in molecular bands, these measurements use variations in the planet radial velocity to disentangle the planet signal from telluric and stellar contaminants. In this paper we apply high resolution spectroscopy to probe molecular absorption in the day-side spectrum of the bright transiting hot Jupiter HD 189733b. We observed HD 189733b with the CRIRES high-resolution near-infrared spectograph on the Very Large Telescope during three nights. We detect a 5-sigma absorption signal from CO at a contrast level of ~4.5e-4 with respect to the stellar continuum, revealing the planet orbital radial velocity at 154+4/-3 km s-1. This allows us to solve for the planet and stellar mass in a similar way as for stellar eclipsing binaries, resulting in Ms= 0.846+0.068/-0.049 Msun and Mp= 1.162+0.058/-0.039 MJup. No significant absorption is detected from H2O, CO2 or CH4 and we determined upper limits on their line contrasts here. The detection of CO in the day-side spectrum of HD 189733b can be made consistent with the haze layer proposed to explain the optical to near-infrared transmission spectrum if the layer is optically thin at the normal incidence angles probed by our observations, or if the CO abundance is high enough for the CO absorption to originate from above the haze. Our non-detection of CO2 at 2.0 micron is not inconsistent with the deep CO2 absorption from low resolution NICMOS secondary eclipse data in the same wavelength range. If genuine, the absorption would be so strong that it blanks out any planet light completely in this wavelength range, leaving no high-resolution signal to be measured.

• The paper details the first definitive detection of CO in the day-side atmosphere of exoplanet HD 189733b using high-resolution spectroscopy with VLT/CRIRES.
• The study achieved a 5-sigma detection of CO absorption at 2.3 μm with a contrast of 4.5x10^-4, enabling precise measurements of the planet's orbital velocity and host star/planet masses.
• The findings, including upper limits for H2O, CO2, and CH4, align with atmospheric haze models and demonstrate high-resolution spectroscopy's potential for detailed exoplanet atmospheric analysis.

Detection of Carbon Monoxide in the Atmosphere of HD 189733b

The paper "Detection of carbon monoxide in the high-resolution day-side spectrum of the exoplanet HD 189733b" presents a notable advancement in the study of exoplanetary atmospheres through high-resolution spectroscopy. A collaborative effort by researchers from institutions including SRON Netherlands Institute for Space Research and Leiden Observatory, the study marks a decisive milestone in detecting molecular absorption in exoplanet atmospheres, focusing on the bright transiting hot Jupiter, HD 189733b.

Research Methodology and Observations

HD 189733b positions itself as a prominent candidate for atmospheric characterization due to its proximity and transiting nature which allows detailed analysis via transit and secondary eclipse events. The researchers applied high-resolution spectroscopy using the CRIRES instrument on the VLT, targeting specific absorption features from molecules including CO, H$_2$O, CH$_4$, and CO$_2$ within 2.0 and 2.3 $\mu$m wavelength ranges. Spanning three nights, the observational data accrued were meticulously analyzed to discern absorption signals amidst telluric and stellar interference.

The 5-$\sigma$ detection of CO absorption achieves a notable contrast level of approximately 4.5$\times$10$^{-4}$ compared to the stellar continuum. This detection enables the determination of the planet’s orbital radial velocity at 154$^{+4}_{-3}$ km s$^{-1}$, and it further facilitates an estimation of the respective masses of the host star (0.846$^{+0.068}_{-0.049}$ $M_{\sun}$) and planet (1.162$^{+0.058}_{-0.039}$ $M_{\mathrm{Jup}$). The results echo the methodologies employed akin to stellar eclipsing binaries, enriching the understanding of transiting exoplanet systems.

Key Findings and Implications

A critical implication of this study is the absence of significant absorption signals from other molecular candidates such as H$_2$O, CO$_2$, and CH$_4$, with only upper limits established for their line contrasts. This absence is consistent with prior secondary eclipse findings, suggesting that high-altitude chemical compositions might not prominently feature these molecules or they exist in less detectable states within the specific spectral windows analyzed.

The robust detection of CO at 2.3 $\mu$m raises implications regarding atmospheric models and chemical equilibrium assumptions within exoplanetary atmospheres. The detected CO absorption aligns with hypotheses involving atmospheric haze layers, such as those proposed by Pont et al., provided the haze remains optically thin or CO abundance is adequately high. This presents a scenario wherein molecular absorptions still occur above or through these aerosols, challenging prior low-resolution estimates that suggested stronger absorptive signals in transmission spectra.

Future Prospects in Exoplanet Atmosphere Studies

The study underscores the pivotal role high-resolution spectroscopy plays in unraveling exoplanet atmospheric dynamics and compositions. The results advocate for continued and systematic observations at diverse spectral ranges, harnessing the capabilities of instruments like CRIRES to refine atmospheric models and understand molecular diversity. Discrepancies between high and low-resolution datasets urge verification through alternate methodologies to corroborate observed molecular features, particularly in contentious cases such as HD 189733b.

As high-resolution spectrography tools and techniques evolve, future observations may further illuminate the atmospheres of non-transiting exoplanets, leveraging radial velocity measurements to predict composition and atmospheric behaviors. The accumulation of such refined datasets will gradually weaken reliance on model assumptions, facilitating empirical determinations of atmospheric compositions and dynamics that could profoundly impact planetary formation and evolutionary theories.

In conclusion, the detection of CO in HD 189733b marks an enhanced capability within exoplanet research, promising avenues for precise characterization of atmospheric components and dynamical properties for this and other distant worlds.