High contrast thermal infrared spectroscopy with ALES: The 3-4µm spectrum of κ andromedae b

We present the first L−band (2.8 to 4.1 µm) spectroscopy of κ Andromedae b, a ∼ 20 M_Jup companion orbiting at 1^′′ projected separation from its B9-type stellar host. We combine our Large Binocular Telescope ALES integral field spectrograph data with measurements from other instruments to analyze the atmosphere and physical characteristics of κ And b. We report a discrepancy of ∼ 20% (2σ) in the L^′ flux of κ And b when comparing to previously published values. We add an additional L^′ constraint using an unpublished imaging dataset collected in 2013 using LBTI/LMIRCam, the instrument in which the ALES module has been built. The LMIRCam measurement is consistent with the ALES measurement, both suggesting a fainter L-band scaling than previous studies. The data, assuming the flux scaling measured by ALES and LMIRCam imaging, are well fit by an L3-type brown dwarf. Atmospheric model fits to measurements spanning 0.9-4.8 µm reveal some tension with the predictions of evolutionary models, but the proper choice of cloud parameters can provide some relief. In particular, models with clouds extending to very-low pressures composed of grains ≤ 1 µm appear to be necessary. If the brighter L^′ photometry is accurate, there is a hint that sub-solar metallicity may be required.