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Atmospheric Characterization of the Nearby Planetary-Mass Companion COCONUTS-2b: Evidence for Disequilibrium Chemistry and Clouds


Core Concepts
Spectroscopic analysis of COCONUTS-2b reveals the presence of disequilibrium chemistry, a diabatic thermal structure, and clouds in its atmosphere, providing insights into the atmospheric properties and formation history of this nearby, widely orbiting planetary-mass companion.
Abstract

Bibliographic Information:

Zhang, Z., Mukherjee, S., Liu, M. C., Fortney, J. J., Mader, E., Best, W. M. J., Dupuy, T. J., Leggett, S. K., Karalidi, T., Line, M. R., Marley, M. S., Morley, C. V., Phillips, M. W., Siverd, R. J., & Zalesky, J. A. (2024). Disequilibrium Chemistry, Diabatic Thermal Structure, and Clouds in the Atmosphere of COCONUTS-2b. The Astronomical Journal, (Submitted).

Research Objective:

This study aims to characterize the atmospheric properties of COCONUTS-2b, a planetary-mass companion orbiting the young M3 star COCONUTS-2A, using newly acquired near-infrared spectroscopy and existing photometric data. The authors investigate the presence of disequilibrium chemistry, a diabatic thermal structure, and clouds in the planet's atmosphere.

Methodology:

The researchers obtained high-quality near-infrared spectra of COCONUTS-2b using the Gemini/Flamingos-2 spectrograph. They compared these spectra with a library of T/Y dwarf spectral templates to refine the object's spectral type. To constrain the atmospheric properties, they conducted an extensive forward-modeling analysis, comparing the observed spectrum and broadband photometry with sixteen state-of-the-art atmospheric model grids for brown dwarfs and self-luminous exoplanets.

Key Findings:

  • The spectral type of COCONUTS-2b is refined to T9.5±0.5.
  • Atmospheric models incorporating disequilibrium chemistry, a diabatic thermal structure, and/or clouds provide the best fit to the observed data.
  • The analysis suggests sub-solar or near-solar metallicity and carbon-to-oxygen ratio (C/O) in COCONUTS-2b's atmosphere.
  • The study determines a bolometric luminosity of log(Lbol/L⊙) = −6.18 dex and derives an effective temperature of Teff = 483+44−53 K, surface gravity of log(g) = 4.19+0.18−0.13 dex, radius of R = 1.11+0.03−0.04 RJup, and mass of M = 8 ± 2 MJup.

Main Conclusions:

The findings indicate that COCONUTS-2b's atmosphere exhibits disequilibrium chemistry, potentially influenced by vertical mixing processes. The presence of a diabatic thermal structure and/or clouds further shapes its atmospheric properties. The derived atmospheric parameters provide valuable constraints for understanding the formation and evolution of this unique planetary-mass companion.

Significance:

This research contributes to the growing body of knowledge about the atmospheric characteristics of wide-orbit planetary-mass companions. The study highlights the importance of considering disequilibrium chemistry, non-adiabatic processes, and cloud formation in modeling the atmospheres of such objects. The findings have implications for understanding the diversity of exoplanet atmospheres and the potential formation pathways of wide-orbit companions.

Limitations and Future Research:

The study acknowledges uncertainties in the alkali chemistry models and opacities, particularly in the Y band. Future observations with JWST and other telescopes will provide more detailed spectroscopic data, enabling further refinement of atmospheric models and a deeper understanding of the chemical and physical processes shaping COCONUTS-2b's atmosphere.

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Stats
COCONUTS-2b is located 10.888 pc from Earth. COCONUTS-2b has a wide orbital separation of 6471 au from its host star. The companion-to-host mass ratio is 0.021±0.005. The age of the COCONUTS-2A+b system is estimated to be 150–800 Myr. The spectral type of COCONUTS-2b is refined to T9.5±0.5. The bolometric luminosity of COCONUTS-2b is log(Lbol/L⊙) = −6.18 dex. The effective temperature of COCONUTS-2b is Teff = 483+44−53 K. The surface gravity of COCONUTS-2b is log(g) = 4.19+0.18−0.13 dex. The radius of COCONUTS-2b is R = 1.11+0.03−0.04 RJup. The mass of COCONUTS-2b is estimated to be M = 8 ± 2 MJup.
Quotes

Deeper Inquiries

How do the atmospheric properties of COCONUTS-2b compare to those of other directly imaged exoplanets and brown dwarfs with similar masses and ages, and what implications do these comparisons have for their formation mechanisms?

COCONUTS-2b, with its ultra-cool atmosphere (T9.5±0.5 spectral type, Teff ~ 483 K), exhibits both similarities and intriguing differences compared to other directly imaged exoplanets and brown dwarfs within a similar mass (~8 ± 2 MJup) and age range (150-800 Myr). Similarities: Disequilibrium Chemistry: Like many late-T and Y dwarfs, COCONUTS-2b shows signs of disequilibrium chemistry, particularly the underabundance of phosphine (PH3). This suggests inefficient vertical mixing in its atmosphere, preventing PH3 dredged up from deeper layers from reaching observable altitudes. This is consistent with the broader trend of decreasing atmospheric mixing efficiency with decreasing temperature observed in brown dwarfs. Potential for Clouds: The study finds that atmospheric models incorporating clouds provide a good fit to COCONUTS-2b's spectrum. This aligns with observations of other late-T and Y dwarfs, where the presence of sulfide and/or water ice clouds is inferred. Differences and Implications for Formation: Metallicity: While the study suggests a sub-solar or near-solar metallicity for COCONUTS-2b, some directly imaged planets like 51 Eri b and HR 8799 planets exhibit super-solar metallicities. This difference could point towards distinct formation pathways. Higher metallicity in some directly imaged planets might favor core accretion, where a planet forms from a metal-rich core that then accretes gas. In contrast, COCONUTS-2b's near-solar metallicity might be more consistent with a gravitational instability origin, where a fragment of the protostellar cloud collapses directly to form the planet. Wide Separation: COCONUTS-2b's exceptionally wide orbit (6471 au) is unusual for both formation scenarios. If formed via core accretion, significant outward migration would be required. If formed through gravitational instability, the wide separation challenges typical models of disk fragmentation. This suggests that COCONUTS-2b's formation might have involved unique dynamical interactions within its birth environment, potentially influenced by the wide separation of the host star binary. Further Research: More precise measurements of COCONUTS-2b's luminosity, age, and dynamical mass are crucial to constrain its formation history and refine comparisons with other substellar objects. Additionally, high-resolution spectroscopy and observations at longer wavelengths with JWST will provide a clearer picture of its atmospheric composition and cloud properties, offering further clues about its formation and evolutionary path.

Could the observed disequilibrium chemistry in COCONUTS-2b's atmosphere be explained by alternative mechanisms other than vertical mixing, such as external processes like photochemistry or accretion of material?

While the study identifies inefficient vertical mixing as the primary explanation for the disequilibrium chemistry observed in COCONUTS-2b's atmosphere, alternative mechanisms could also contribute to these chemical imbalances. Alternative Mechanisms: Photochemistry: Ultraviolet (UV) radiation from the host star, though weaker at COCONUTS-2b's wide separation, could still drive photochemical reactions in the upper atmosphere. These reactions can alter the abundances of certain molecules, leading to deviations from chemical equilibrium. The efficiency of photochemistry depends on the UV spectrum of the host star and the atmospheric composition of COCONUTS-2b, which are still not fully constrained. Accretion of Material: COCONUTS-2b's wide orbit might make it susceptible to accreting material from the interstellar medium or remnants of the protoplanetary disk. This external influx of material, potentially with a different chemical composition, could perturb the atmospheric chemistry and contribute to the observed disequilibrium. The rate of accretion onto COCONUTS-2b is uncertain and would depend on the density of the surrounding environment. Distinguishing Mechanisms: Disentangling the contributions of vertical mixing, photochemistry, and accretion requires detailed modeling that considers the interplay of these processes. Observing the abundances of specific molecules sensitive to different chemical pathways can provide clues. For instance, detecting molecules primarily produced through photochemistry would strengthen the case for its role in shaping COCONUTS-2b's atmospheric composition. Future Observations: High-resolution spectroscopy with instruments like JWST can provide more precise abundance measurements of a wider range of molecules, enabling a more comprehensive assessment of the chemical processes at play. Additionally, studying the variability of COCONUTS-2b's atmospheric properties over time could help distinguish between internal processes like vertical mixing and external influences like accretion.

Given the wide orbital separation of COCONUTS-2b, what are the potential implications of its atmospheric properties for the long-term evolution and stability of the COCONUTS-2 system?

COCONUTS-2b's wide orbital separation, coupled with its intriguing atmospheric properties, raises interesting questions about the long-term evolution and stability of the COCONUTS-2 system. Implications for Evolution and Stability: Reduced Tidal Interactions: The vast distance between COCONUTS-2b and its host star significantly diminishes tidal forces. This implies that COCONUTS-2b's orbital parameters, such as its semi-major axis and eccentricity, are likely to remain relatively stable over long timescales. This stability contrasts with planets in close-in orbits, which experience strong tidal interactions that can lead to orbital decay and eventual engulfment by the host star. Atmospheric Escape: Despite its cool temperature, COCONUTS-2b's low surface gravity makes it susceptible to atmospheric escape. The efficiency of escape mechanisms like Jeans escape and photoevaporation depends on the atmospheric temperature, composition, and the high-energy radiation environment around the host star. While the wide separation reduces the impact of the stellar wind, the potential for atmospheric loss over billions of years cannot be ruled out. Dynamical Stability: The long-term stability of wide-orbit systems like COCONUTS-2 can be influenced by external factors such as encounters with passing stars or the galactic tidal field. These perturbations can potentially disrupt the system's architecture, leading to scattering or ejection of the planet. However, the probability of such events is generally low, especially for systems residing in relatively calm galactic environments. Future Studies: Long-Term Monitoring: Observing COCONUTS-2b's orbit over decades or even centuries can provide insights into its dynamical stability and potential for orbital migration. Atmospheric Escape Models: Detailed modeling of atmospheric escape processes, considering COCONUTS-2b's specific properties and the host star's radiation environment, can help assess the potential for atmospheric loss over its lifetime. Population Studies: Comparing COCONUTS-2b's properties and orbital configuration with other wide-orbit systems can shed light on the prevalence and long-term evolution of such systems in the galactic context. Understanding the interplay between COCONUTS-2b's atmospheric properties, its wide orbit, and external influences is crucial for unraveling the long-term fate of this intriguing planetary system.
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