Deep Pa$β$ Imaging of the Candidate Accreting Protoplanet AB Aur b

Abstract: Giant planets grow by accreting gas through circumplanetary disks, but little is known about the timescale and mechanisms involved in the planet assembly process because few accreting protoplanets have been discovered. Recent visible and infrared (IR) imaging revealed a potential accreting protoplanet within the transition disk around the young intermediate-mass Herbig Ae star, AB Aurigae (AB Aur). Additional imaging in H$\alpha$ probed for accretion and found agreement between the line-to-continuum flux ratio of the star and companion, raising the possibility that the emission source could be a compact disk feature seen in scattered starlight. We present new deep Keck/NIRC2 high-contrast imaging of AB Aur to characterize emission in Pa$\beta$, another accretion tracer less subject to extinction. Our narrow band observations reach a 5$\sigma$ contrast of 9.6 mag at 0.6$''$, but we do not detect significant emission at the expected location of the companion, nor from other any other source in the system. Our upper limit on Pa$\beta$ emission suggests that if AB Aur b is a protoplanet, it is not heavily accreting or accretion is stochastic and was weak during the observations.