Testing Young Brown Dwarf Atmospheric Properties with L-Band Spectroscopy

One of the greatest challenges in the study of directly-imaged imaged exoplanets has been the determination of their atmospheric properties from observed spectroscopy and photometry.  In particular, the extremely red near-IR colors and lack of methane for exoplanets have been difficult to model. There are several explanations for these properties of directly-imaged exoplanets, such as thermo-chemical instability, disequilibrium chemistry, and dust clouds.  The difficulty in observing exoplanet atmospheres, however, have limited the wavelength coverage and detail with which these theories have been tested. With the intent to test these theories, we use young brown dwarf spectra as proxies for these exoplanet spectra. The objects we have chosen are of spectral types, masses, and ages that overlap with the red directly-imaged exoplanets. Specifically we are looking at the L-band where the 3.3 micron methane feature provides a sensitive probe for disequilibrium chemistry and the spectral slope probes for dust-grain size. We detect methane at spectral type L4 and later, which matches the onset of methane in field brown dwarfs. This work showcases the diagnostic power of L-band spectroscopy and the potential for observations with future facilities (such as JWST) to aid in our understanding of planetary atmospheres.