We obtained spectra of the pre-main sequence star AU Microscopii during a transit of its Neptune-sized planet to investigate its orbit and atmosphere. We used the high-dispersion near-infrared spectrograph IRD on the Subaru telescope to detect the Doppler "shadow" from the planet and constrain the projected stellar obliquity. Modeling of the observed planetary Doppler shadow suggests a spin-orbit alignment of the system (λ=−4.7−6.4+6.8 degrees), but additional observations are needed to confirm this finding. We use both the IRD data and spectra obtained with NIRSPEC on Keck-II to search for absorption in the 1083 nm line of metastable triplet He I by the planet's atmosphere and place 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 escape rate from the atmosphere to <0.15−0.45M⊕ Gyr−1, comparable to the rates predicted by an XUV energy-limited escape calculation and hydrodynamic models, but refinement of the planet mass is needed for rigorous tests.
@article{arxiv.2006.13243,
title = {Limits on the Spin-Orbit Angle and Atmospheric Escape for the 22 Myr-old Planet AU Mic b},
author = {Teruyuki Hirano and Vigneshwaran Krishnamurthy and Eric Gaidos and Heather Flewelling and Andrew W. Mann and Norio Narita and Peter Plavchan and Takayuki Kotani and Motohide Tamura and Hiroki Harakawa and Klaus Hodapp and Masato Ishizuka and Shane Jacobson and Mihoko Konishi and Tomoyuki Kudo and Takashi Kurokawa and Masayuki Kuzuhara and Jun Nishikawa and Masashi Omiya and Takuma Serizawa and Akitoshi Ueda and Sébastien Vievard},
journal= {arXiv preprint arXiv:2006.13243},
year = {2020}
}
Comments
10 pages, 5 figures, accepted for publication in ApJ Letters