High Contrast Thermal Infrared Spectroscopy with ALES: The 3-4$\hat{I}¼$m Spectrum of $\hat{I}º$ Andromedae b

Abstract

We present the first $L-$band (2.8 to 4.1~$\hat{I}¼$m) spectroscopy of $\hat{I}º$~Andromedae~b, a $\sim 20M_{\mathrm{Jup}}$ companion orbiting at $1^{\prime \prime }$ 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 $\hat{I}º$~And~b. We report a discrepancy of $\sim 20\%$ ($2\ddot{I}ƒ$) in the $L^{\prime }$ flux of $\hat{I}º$~And~b when comparing to previously published values. We add an additional $L^{\prime }$ constraint using an unpublished imaging dataset collected in 2013 using LBTI/LMIRCam, the instrument in which the ALES module has been built.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~$\hat{I}¼$m 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 $\le 1\hat{I}¼$m appear to be necessary. If the brighter $L^{\prime }$ photometry is accurate, there is a hint that sub-solar metallicity may be required.

Accepted for publication in AJ