2016ApJ...829...39S

L-band spectroscopy is a powerful probe of cool low-gravity atmospheres: the P, Q, and R branch fundamental transitions of methane near 3.3 µm provide a sensitive probe of carbon chemistry; cloud thickness modifies the spectral slope across the band; and {
H}_{3}^+ opacity can be used to detect aurorae. Many directly imaged gas-giant companions to nearby young stars exhibit L-band fluxes distinct from the field population of brown dwarfs at the same effective temperature. Here we describe commissioning the L-band spectroscopic mode of Clio2, the 1-5 µm instrument behind the Magellan adaptive-optics system. We use this system to measure L-band spectra of directly imaged companions. Our spectra are generally consistent with the parameters derived from previous near-infrared spectra for these late M to early L type objects. Therefore, deviations from the field sequence are constrained to occur below 1500 K. This range includes the L-T transition for field objects and suggests that observed discrepancies are due to differences in cloud structure and CO/CH_4 chemistry.