2025ApJ...990L..42C

We analyze high-contrast, medium-spectral-resolution H_α observations of the star AB Aurigae using the Very Large Telescope's Multi Unit Spectroscopic Explorer (MUSE). In multiple epochs, MUSE detects the AB Aur b protoplanet discovered from Subaru/SCExAO data in emission at wavelengths slightly blueshifted from the H_α line center (i.e., at 6558.88-6560.13Å; ∼-100 km s^–1) and in absorption at redshifted wavelengths (6562.8-6565.1Å; ∼75 km s^–1). AB Aur b's H_α spectrum is inconsistent with that of the host star or the average residual disk spectrum and is dissimilar to that of PDS 70 b and c. Instead, the spectrum's shape resembles that of an inverse P Cygni profile seen in some accreting T Tauri stars and interpreted as evidence of infalling cold gas from accretion, although we cannot formally rule out all other nonaccretion origins for AB Aur b's MUSE detection. AB Aurigae hosts only the second protoplanetary system detected in H_α thus far and the first with a source showing a spectrum resembling an inverse P Cygni profile. Future modeling and new optical data will be needed to assess how much AB Aur b's emission source(s) originates from protoplanet accretion reprocessed by the disk, a localized scattered-light feature with a unique H_α profile, or another mechanism.