An L Band Spectrum of the Coldest Brown Dwarf

Abstract: The coldest brown dwarf, WISE 0855, is the closest known planetary-mass, free-floating object and has a temperature nearly as cold as the solar system gas giants. Like Jupiter, it is predicted to have an atmosphere rich in methane, water, and ammonia, with clouds of volatile ices. WISE 0855 is faint at near-infrared wavelengths and emits almost all its energy in the mid-infrared. Skemer et al. 2016 presented a spectrum of WISE 0855 from 4.5-5.1 micron (M band), revealing water vapor features. Here, we present a spectrum of WISE 0855 in L band, from 3.4-4.14 micron. We present a set of atmosphere models that include a range of compositions (metallicities and C/O ratios) and water ice clouds. Methane absorption is clearly present in the spectrum. The mid-infrared color can be better matched with a methane abundance that is depleted relative to solar abundance. We find that there is evidence for water ice clouds in the M band spectrum, and we find a lack of phosphine spectral features in both the L and M band spectra. We suggest that a deep continuum opacity source may be obscuring the near-infrared flux, possibly a deep phosphorous-bearing cloud, ammonium dihyrogen phosphate. Observations of WISE 0855 provide critical constraints for cold planetary atmospheres, bridging the temperature range between the long-studied solar system planets and accessible exoplanets. JWST will soon revolutionize our understanding of cold brown dwarfs with high-precision spectroscopy across the infrared, allowing us to study their compositions and cloud properties, and to infer their atmospheric dynamics and formation processes.

• The paper establishes a robust detection of methane absorption in WISE 0855's L band spectrum, suggesting a subsolar methane abundance.
• The paper identifies water ice clouds that flatten spectral features, significantly impacting the thermal emission of this ultra-cool brown dwarf.
• The paper highlights an unexplained near-infrared flux discrepancy, indicating a deep continuum opacity source that could imply novel cloud formation processes.

Insightful Overview of "An L Band Spectrum of the Coldest Brown Dwarf"

The study detailed in this paper presents a comprehensive analysis of the coldest known brown dwarf, WISE 0855, which possesses characteristics remarkably similar to the gas giants in our Solar System. As the closest identified substellar object of its temperature range, WISE 0855 offers an exceptional opportunity to explore cold planetary atmospheres in detail. This paper contributes significantly to our understanding of such atmospheric compositions through the presentation of its L band spectrum (3.4-4.14 $\mu$m) and the development of nuanced atmospheric models.

Key Findings and Analysis

1. Methane in WISE 0855: The detection of methane (CH$_4$) absorption in WISE 0855 is robustly confirmed via spectral analysis. The methane features dominate the L band spectrum, aligning well with theoretical predictions for cold, Jupiter-like atmospheres. Notably, the authors propose that the observed mid-infrared spectrum of WISE 0855 is compatible with a reduced methane abundance relative to solar expectations. This indicates either a primordial compositional difference (potentially a lower metallicity or non-solar C/O ratio) or the existence of a presently underestimated atmospheric process.
2. Water Ice Clouds: The presence of water ice clouds within the atmosphere of WISE 0855 is supported by the flattened and muted features of the brown dwarf's M band spectrum (4.5-5.1 $\mu$m). The paper underscores that clouds significantly impact the thermal emission of such cold atmospheres, distinguishing the absorptive properties across different wavelengths. This finding aligns with earlier theoretical studies on water cloud formation in cooler brown dwarfs, emphasizing their spectral implications.
3. Phosphine Absence: Contrary to the atmosphere of Jupiter, WISE 0855 exhibits no discernible phosphine (PH$_3$) absorption, despite phosphine being a significant constituent in Jovian atmospheric chemistry. This absence suggests distinct chemical dynamics or reduced phosphorous availability, further discussed in terms of potential deep cloud formations or other scapegoat chemical sinks in the atmosphere.
4. Continuum Opacity Sources: The analysis highlights a discrepancy in the near-infrared flux which the authors hypothesize to be due to an unknown deep opacity source, potentially at pressures around 8-10 bar. The implications of such a source include reduced visible and infrared flux, without affecting the mid-infrared spectrum. This could imply novel cloud formation processes or unique chemical interactions, potentially involving ammonium dihydrogen phosphate clouds.

Implications for Astrophysical Research

The research presented not only provides fresh insights into the composition and behavior of ultra-cool substellar atmospheres but broadens the framework for interpreting exoplanetary data as well. The authors project an intriguing outlook with the imminent capabilities of the JWST, positing that it will elucidate several of the undecided aspects highlighted in this study. In particular, JWST should adeptly pinpoint the detailed atmospheric composition including the anticipated trace gases like phosphine and CO-related compounds, enhancing our comprehension of Y dwarf and cold exoplanet climates.

Theoretical and Future Developments

The observations and models from this study have significant ramifications for atmospheric dynamics in cold, Jupiter-like exoplanets. Future work should focus deeply on spectrally resolving potential discrepancies using comprehensive radiative-convective models, potentially incorporating atmospheric circulation dynamics and microphysical cloud models to understand cloud formation better. Furthermore, studies should explore non-equilibrium chemistry processes to account for the low methane absorptive properties across WISE 0855 and similar objects.

Conclusion

This paper robustly tackles the spectral analysis of WISE 0855, offering crucial advancements in our comprehension of cold brown dwarf atmospheres. By presenting pioneering atmospheric models in conjunction with observational data, the study carves a path for future explorations and theoretical work on substellar and exoplanetary atmospheres in similar thermal regimes. These findings act as a preparatory scaffold for upcoming JWST insights, expected to unlock further mysteries of these fascinating celestial neighbors.