H$\alpha$ Variability of AB Aur b with the Hubble Space Telescope: Probing the Nature of a Protoplanet Candidate with Accretion Light Echoes

TL;DR

This study introduces a novel method using accretion light echoes to distinguish between true accreting protoplanets and disk artifacts in H$ extalpha$ observations, applied to the candidate AB Aur b with Hubble data.

Contribution

The paper presents a new observational technique leveraging variable H$ extalpha$ emission to identify accreting protoplanets, demonstrated through Hubble observations of AB Aur b.

Findings

AB Aur b shows significant H$ extalpha$ variability uncorrelated with its host star.

The variability suggests AB Aur b is an independently accreting protoplanet.

The method can differentiate true protoplanets from scattered disk features.

Abstract

Giant planets generate accretion luminosity as they form. Much of this energy is radiated in strong H$\alpha$ line emission, which has motivated direct imaging surveys at optical wavelengths to search for accreting protoplanets. However, compact disk structures can mimic accreting planets by scattering emission from the host star. This can complicate the interpretation of H$\alpha$ point sources, especially if the host star itself is accreting. We describe an approach to distinguish accreting protoplanets from scattered-light disk features using "accretion light echoes." This method relies on variable H$\alpha$ emission from a stochastically accreting host star to search for a delayed brightness correlation with a candidate protoplanet. We apply this method to the candidate protoplanet AB Aur b with a dedicated Hubble Space Telescope Wide Field Camera 3 program designed to sequentially sample the host star and the candidate planet in H$\alpha$ while accounting for the light travel time delay and orbital geometry of the source within the protoplanetary disk. Across five epochs spanning 14 months, AB Aur b is over 20 times more variable than its host star; AB Aur's H$\alpha$ emission changes by 15% while AB Aur b varies by 330%. These brightness changes are not correlated, which rules out unobstructed scattered starlight from the host star as the only source of AB Aur b's H$\alpha$ emission and is consistent with tracing emission from an independently accreting protoplanet, inner disk shadowing effects, or a physically evolving compact disk structure. More broadly, accretion light echoes offer a novel tool to explore the nature of protoplanet candidates with well-timed observations of the host star prior to deep imaging in H$\alpha$.