The First Spectrum of the Coldest Brown Dwarf

Abstract: The recently discovered brown dwarf WISE 0855 presents our first opportunity to directly study an object outside the Solar System that is nearly as cold as our own gas giant planets. However the traditional methodology for characterizing brown dwarfs---near infrared spectroscopy---is not currently feasible as WISE 0855 is too cold and faint. To characterize this frozen extrasolar world we obtained a 4.5-5.2 $\mu$m spectrum, the same bandpass long used to study Jupiter's deep thermal emission. Our spectrum reveals the presence of atmospheric water vapor and clouds, with an absorption profile that is strikingly similar to Jupiter. The spectrum is high enough quality to allow the investigation of dynamical and chemical processes that have long been studied in Jupiter's atmosphere, but now on an extrasolar world.

• The paper presents the first detailed spectral analysis of WISE 0855, revealing strong water vapor absorption and atmospheric similarities with Jupiter.
• The study employed Gemini-North GNIRS over 13 nights in the 4.5–5.2 μm band to overcome challenges posed by the brown dwarf's faintness.
• The absence of phosphine signals and the inferred water cloud composition challenge existing atmospheric models and guide future exoplanet research.

Analysis of Cold Brown Dwarf Spectroscopy: Case Study of WISE 0855

The study at hand provides an unprecedented examination of the brown dwarf WISE 0855, an object with temperatures analogous to those of the Solar System's gas giants, particularly Jupiter. WISE 0855 is notable for being the coldest known brown dwarf and nearest known planetary mass object beyond our Solar System, exhibiting a temperature near 250 K. The main objective of this paper is the spectral analysis in the 4.5-5.2 $\mu$m range, capturing insights into its atmospheric composition and structure in a manner similar to studies of Jupiter's deep thermal emissions.

Observational Constraints and Methodology

Conducting spectral analysis on such a faint and cold object posed significant challenges, primarily due to its dimness in traditional near-infrared spectrometry. WISE 0855's unique characteristics necessitated the use of a specific spectral band where water vapor absorption is prominent, exploiting the atmospheric window between 4.5-5.2 $\mu$m where opacity from $\rm CH_{4}$ and $\rm H_{2}$ is minimized. The Gemini-North telescope, equipped with the GNIRS, facilitated these observations, spanning 13 nights with a total accumulation of 14.4 hours of data. A meticulous calibration process, including telluric corrections, yielded a high-fidelity spectral dataset.

Key Findings and Theoretical Implications

The gathered spectrum reveals striking similarities between WISE 0855 and Jupiter, particularly in the strong presence of atmospheric water vapor. This similarity extends to the dynamical and chemical processes that can now be inferred with empirical data, previously restricted largely to Jupiter studies. The notable absence of phosphine ($\rm PH_{3}$) signals suggests differences in atmospheric mixing processes compared to Jupiter, potentially attributed to weaker turbulent mixing or differing chemical equilibria.

Further insights were gleaned into the potential cloud composition, with water clouds hypothesized to be a significant component due to the observed spectral profile. The observed features are consistent with atmospheric models predicting water vapor absorption, and the analysis suggests clouds influence the absorption characteristics significantly.

Implications and Future Directions

The implications of this research are manifold. From a practical standpoint, this approach advances the capability to study other similarly cold astronomical objects, extending potential applications even to the further reaches of exoplanetary systems. Theoretically, it challenges existing models of atmospheric dynamics, demanding an incorporation of cloud effects and mixing dynamics to match observations more effectively.

Looking forward, the study of WISE 0855 sets the stage for future spectral examinations using next-generation telescopes, such as the James Webb Space Telescope and future ELTs, to gain comprehensive insights into substellar objects of similar temperatures. These facilities promise to extend the reach of such studies beyond current capabilities, potentially including much cooler objects and providing deeper comprehension of planetary formation and atmospheric evolution processes.

In conclusion, the study of WISE 0855 not only offers invaluable data for comparisons with objects within our Solar System but also inspires refined models of atmospheric science in planetary science and beyond, bridging the detailed study of brown dwarfs and cold exoplanets. The methodologies and findings provide a critical reference point for future research endeavors in these domains.