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We obtained spectra of the pre-main sequence star AU Microscopii during atransit of its Neptune-sized planet to investigate its orbit and atmosphere. Weused the high-dispersion near-infrared spectrograph IRD on the Subaru telescopeto detect the Doppler "shadow" from the planet and constrain the projectedstellar obliquity. Modeling of the observed planetary Doppler shadow suggests aspin-orbit alignment of the system ($\lambda=-4.7_{-6.4}^{+6.8}$ degrees), butadditional observations are needed to confirm this finding. We use both the IRDdata and spectra obtained with NIRSPEC on Keck-II to search for absorption inthe 1083 nm line of metastable triplet He I by the planet's atmosphere andplace an upper limit for the equivalent width of 3.7 m\AA at 99 $\%$confidence. With this limit and a Parker wind model we constrain the escaperate from the atmosphere to $<0.15-0.45\, M_{\oplus}$ Gyr$^{-1}$, comparable tothe rates predicted by an XUV energy-limited escape calculation andhydrodynamic models, but refinement of the planet mass is needed for rigoroustests.

Limits on the Spin-Orbit Angle and Atmospheric Escape for the 22 Myr-old Planet AU Mic b